Honeycomb filter

ABSTRACT

A honeycomb filter includes a plurality of cells and porous cell walls. The plurality of cells include exhaust gas introduction cells and exhaust gas emission cells. Each of the exhaust gas emission cells is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls. The exhaust gas introduction cells include first exhaust gas introduction cells and second exhaust gas introduction cells. Each of the first exhaust gas introduction cells has the cross sectional area equal to or smaller than 0.7 mm 2  in the cross section perpendicular to the longitudinal direction of the plurality of cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-159938, filed Jul. 31, 2013. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a honeycomb filter.

2. Background Art

Particulates (hereinafter also referred to as PMs) such as soot in exhaust gas discharged from internal combustion engines including diesel engines cause damage to environment and human bodies, and these days people have paid attention to this problem. Since exhaust gas contains toxic gas components such as CO, HC, and NOx, people also worry about the influences of the toxic gas components on the environment and human bodies.

To overcome this problem, various filters having honeycomb structures (honeycomb filters) formed of porous ceramics such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatus. Such honeycomb filters are connected to internal combustion engines to capture PMs in exhaust gas, or to convert the toxic gas components such as CO, HC, and NOx in the exhaust gas into nontoxic gas.

For enhancing the fuel economy of internal combustion engines and avoiding troubles derived from an increase in the pressure loss during operation, various honeycomb filters have been proposed including those in which the initial pressure loss is decreased by improvement of the cell structure and those in which the rate of increase in the pressure loss is low when a certain amount of PM accumulate.

Such honeycomb filters are disclosed for example in WO 2004/024294 and U.S. Pat. No. 4,417,908.

FIG. 17A is a perspective view schematically illustrating a honeycomb filter disclosed in WO 2004/024294. FIG. 17B is a perspective view schematically illustrating a honeycomb fired body forming the honeycomb filter.

As shown in FIGS. 17A and 17B, WO 2004/024294 discloses a honeycomb filter 90 that includes a plurality of honeycomb fired bodies 100 combined with one another with adhesive layers 105 residing therebetween, and an periphery coat layer 106 formed on the periphery of the combined honeycomb fired bodies, wherein the honeycomb fired bodies 100 each include exhaust gas introduction cells 102 each having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side, and exhaust gas emission cells 101 each having an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side; the exhaust gas emission cells 101 each have a square cross section perpendicular to the longitudinal direction of the cells; the exhaust gas introduction cells 102 each have an octagonal cross section perpendicular to the longitudinal direction of the cells; and the exhaust gas emission cells 101 and the exhaust gas introduction cells 102 are alternately (in a grid-like pattern) arranged.

Hereinafter, a cell having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side as described herein is simply described as an exhaust gas emission cell. Moreover, a cell having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side is simply described as an exhaust gas introduction cell, a first exhaust gas introduction cell, or a second exhaust gas introduction cell.

The term just described as “cell” means both of the exhaust gas emission cell and exhaust gas introduction cell.

Moreover, a cross section perpendicular to the longitudinal direction of cells including exhaust gas introduction cells, exhaust gas emission cells, or the like is simply described as a cross section of the exhaust gas introduction cells, exhaust gas emission cells, or the like.

FIG. 18A is a perspective view schematically illustrating a honeycomb filter disclosed in U.S. Pat. No. 4,417,908. FIG. 18B is a view schematically illustrating an end face of the honeycomb filter.

U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which all cells have the same square cross-sectional shape as shown in FIGS. 18A and 18B. In the honeycomb filter 110, exhaust gas emission cells 111 each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side are adjacently surrounded fully by exhaust gas introduction cells 112 and 114 each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side across cell walls 113. In the cross section, a side of the exhaust gas introduction cell 112 faces the exhaust gas emission cell 111 across the cell wall 113, whereas the corners of the exhaust gas introduction cells 114 respectively face the corners of the exhaust gas emission cells 111. Thus, none of the sides forming the cross sections of the exhaust gas introduction cells 114 faces the exhaust gas emission cells 111.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a honeycomb filter includes a plurality of cells and porous cell walls. Exhaust gas is to flow through the plurality of cells. The plurality of cells include exhaust gas introduction cells and exhaust gas emission cells. The exhaust gas introduction cells each have an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side. The exhaust gas emission cells each have an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side. The exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for a plugged portion in a cross section perpendicular to the longitudinal direction of the plurality of cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side. The porous cell walls define rims of the plurality of cells. Each of the exhaust gas emission cells is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls. The exhaust gas introduction cells include first exhaust gas introduction cells and second exhaust gas introduction cells. Each of the second exhaust gas introduction cells has a cross sectional area larger than a cross sectional area of each of the first exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells. Each of the exhaust gas emission cells has a cross sectional area equal to or larger than the cross sectional area of each of the second exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells. Each of the first exhaust gas introduction cells has the cross sectional area equal to or smaller than 0.7 mm² in the cross section perpendicular to the longitudinal direction of the plurality of cells.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 is an enlarged end face view illustrating an enlarged image of a part of an end face of a honeycomb filter according to one embodiment of the present invention.

FIGS. 2A to 2C are each an enlarged end face view illustrating an enlarged image of a part of an end face of a honeycomb filter according to one embodiment of the present invention.

FIG. 3 is a perspective view schematically illustrating one example of the honeycomb filter according to a first embodiment of the present invention.

FIG. 4A is a perspective view schematically illustrating one example of the honeycomb fired body forming the honeycomb filter shown in FIG. 3. FIG. 4B is an A-A line cross sectional view of the honeycomb fired body shown in FIG. 4A.

FIG. 5 is an enlarged cross sectional view perpendicular to the longitudinal direction of the honeycomb filter. FIG. 5 illustrates how each cell unit (cell structure) is two-dimensionally, i.e. in X and Y directions, repeated in the case where the second exhaust gas introduction cells and the exhaust gas emission cells are octagonal and the first exhaust gas introduction cells are square in the cross section of the cells, and also illustrates how the first exhaust gas introduction cells and the second exhaust gas introduction cells are shared between the cell units (cell structure).

FIG. 6 is an end face view illustrating an end face of the honeycomb filter according to the first embodiment of the present invention.

FIG. 7 is an explanatory view schematically illustrating a method for measuring the pressure loss.

FIG. 8 is a graph showing a relation between PM capture amount and the pressure loss that are measured in Example 1 and Comparative Example 1.

FIG. 9A is a perspective view schematically illustrating one example of the honeycomb filter according to a second embodiment of the present invention. FIG. 9B is a B-B line cross sectional view of the honeycomb filter shown in FIG. 9A.

FIG. 10A is a perspective view schematically illustrating one example of the honeycomb filter according to a third embodiment of the present invention. FIG. 10B is a perspective view schematically illustrating the honeycomb fired body forming the honeycomb filter.

FIG. 11A is a perspective view illustrating one modified example of a honeycomb fired body forming the honeycomb filter according to a third embodiment of the present invention. FIG. 11B is an end face view of the honeycomb filter fired body shown in FIG. 11A. FIG. 11C is an end face view illustrating another modified example of a honeycomb fired body forming the honeycomb filter according to the third embodiment of the present invention.

FIG. 12A is an end face view schematically illustrating one example of the cell arrangement at an end face of a honeycomb fired body forming the honeycomb filter according to a fourth embodiment of the present invention. FIG. 12B is an end face view of another modified example of a honeycomb fired body forming the honeycomb filter according to the fourth embodiment of the present invention. FIG. 12C is an end face view of still another modified example of a honeycomb fired body forming the honeycomb filter according to the fourth embodiment of the present invention.

FIG. 13 is an enlarged cross sectional view perpendicular to the longitudinal direction of the honeycomb filter. FIG. 13 illustrates how each cell unit (cell structure) is two-dimensionally, i.e. in X and Y directions, repeated in the case where the first exhaust gas introduction cells, the second exhaust gas introduction cells, and the exhaust gas emission cells are square in the cross section of the cells, and also illustrates how the first exhaust gas introduction cells and the second exhaust gas introduction cells are shared between the cell units (cell structure).

FIG. 14 is an end face view schematically illustrating one example of the cell arrangement in an end face of a honeycomb fired body forming the honeycomb filter according to a fifth embodiment of the present invention.

FIG. 15 is an end face view schematically illustrating one example of the cell arrangement at an end face of the honeycomb fired body forming the honeycomb filter according to the sixth embodiment of the present invention.

FIG. 16A is an explanatory view schematically illustrating one example of a convex square cell shape. FIG. 16B is an explanatory view schematically illustrating one example of a concave square cell shape. FIG. 16C is an explanatory side view schematically illustrating one example of the concave square shape in which a vertex portion is chamfered. FIG. 16D is an explanatory view schematically illustrating one example of the convex square shape in which a vertex portion is chamfered.

FIG. 17A is a perspective view schematically illustrating a honeycomb filter disclosed in WO 2004/024294, FIG. 17B is a perspective view schematically illustrating a honeycomb fired body forming the honeycomb filter.

FIG. 18A is a perspective view schematically illustrating a honeycomb filter disclosed in U.S. Pat. No. 4,417,908. FIG. 18B is an end face view schematically illustrating an end face of the honeycomb filter.

FIG. 19A is a perspective view schematically illustrating a honeycomb filter according to comparative examples. FIG. 19B is a perspective view schematically illustrating a honeycomb fired body forming the honeycomb filter shown in FIG. 19A.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention provide a honeycomb filter including a plurality of cells for allowing exhaust gas to flow therethrough, the cells including exhaust gas introduction cells and exhaust gas emission cells, and porous cell walls defining rims of the plurality of cells,

the exhaust gas introduction cells each having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side,

the exhaust gas emission cells each having an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side,

the exhaust gas introduction cells and the exhaust gas emission cells each having a uniform cross sectional shape except for the plugged portion in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side,

wherein each exhaust gas emission cell is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls,

the exhaust gas introduction cells include first exhaust gas introduction cells and second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells,

each exhaust gas emission cell has a cross sectional area that is equal to or larger than the cross sectional area of each second exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells, and

each first exhaust gas introduction cell has a cross sectional area of 0.7 mm² or smaller in a direction perpendicular to the longitudinal direction of the cells.

In the honeycomb filter of the embodiments of the present invention, each first exhaust gas introduction cells has a cross sectional area of 0.7 mm² or smaller in a direction perpendicular to the longitudinal direction of the cells. Consequently, the opening of each first exhaust, gas introduction cell becomes small as the amount of PMs accumulated in the honeycomb filter reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. At the same time, as described below, the passage resistance upon passage of exhaust gas through the exhaust gas emission cells as well as the outflow resistance upon emission of exhaust gas from the honeycomb filter are small because the cross sectional area of each exhaust gas emission cell is large; and PMs accumulate widely and thinly in the first exhaust gas introduction cells and the second exhaust gas introduction cells because the cross sectional area of each second exhaust gas introduction cell is larger than the cross sectional area of each first exhaust gas introduction cell. Owing to such a structure, the pressure loss can be maintained small until the amount of accumulated PMs reaches a level where the pressure loss rapidly increases (i.e., until the amount of accumulated PM is 6 to 10 g/L). As described above, PMs accumulate in substantially uniform thickness also in the inner walls of the exhaust gas introduction cells having different cross sectional areas (i.e., the first exhaust gas introduction cells and the second exhaust gas introduction cells). By forming some of the exhaust gas introduction cells to have small cross sectional areas, it is possible to create a situation where the openings on the exhaust gas introduction side of these some of the exhaust gas introduction cells are substantially blocked once the amount of accumulated PMs reaches 6 to 10 g/L, thus allowing a rapid increase in the pressure loss. Consequently, the regeneration treatment can be reliably carried out at an appropriate time. Thus, in the honeycomb filter of the embodiments of the present invention, the excessive capture and the increase in the amount of heat generated by regeneration are suppressed, and the honeycomb filter is thus less likely to have cracks and dissolution loss.

The reason as to why the pressure loss can be maintained small until the amount of PMs accumulated in the honeycomb filter reaches 6 to 10 g/L is explained in detail below.

The inventors of the present application see that the pressure loss occurs due to (a) inflow resistance caused by exhaust gas flowing into the honeycomb filter, (b) flow-through resistance in the exhaust gas introduction cells, (c) passage resistance in the cell walls, (d) passage resistance caused by exhaust gas upon passing through a layer of accumulated PMs, (e) flow-through resistance in the exhaust gas emission cells, and (f) outflow resistance caused by exhaust gas flowing out of the honeycomb filter. The study of the inventors has revealed that the factors (c), (e), and (f) are controlling factors of the initial pressure loss that occurs before accumulation of PMs, and that the factors (a), (b), and (d) are controlling factors of the transitional pressure loss that occurs after accumulation of a certain amount of PMs. One of the controlling factors of the initial pressure loss is not the factor (b) flow-through resistance in the exhaust gas introduction cells but the factor (e) flow-through resistance in the exhaust gas emission cells because the aperture ratio of the honeycomb filter based on the exhaust gas emission cells is smaller than the aperture ratio of the honeycomb filter based on the exhaust gas introduction cells. Similarly, the inventors consider the factor (f) outflow resistance caused by exhaust gas flowing out of the honeycomb filter, not the factor (a) inflow resistance caused by exhaust gas flowing into the honeycomb filter, as one of the controlling factors of the initial pressure loss because they suppose that the resistance due to compression of the gas is smaller than the resistance due to disturbance of emission of the exhaust gas caused by eddying flow of the gas that occurs near the emission end when the gas rapidly expands upon emission from the cells.

In order to reduce the initial pressure loss, it is necessary to decrease the passage resistance and the outflow resistance to exhaust gas. Thus, in order to suppress rapid expansion, the cross sectional area of each exhaust gas emission cell must be equal to or larger than that of the exhaust gas introduction cell. At the same time, in order to reduce the excessive pressure loss, it is necessary to allow PMs to accumulate widely and thinly. Thus, the cross sectional area of each exhaust gas introduction cell must be larger than that of the exhaust gas emission cell. In other words, it is necessary to employ two types of exhaust gas introduction cells, one having a large cross sectional area (i.e., the second exhaust gas introduction cell) and one having a small cross sectional area (i.e., the first exhaust gas introduction cells), and to employ exhaust gas emission cells each of which has a cross sectional area that is equal to or larger than that of each second exhaust gas introduction cell. In the honeycomb filter of the embodiments of the present invention, the exhaust gas introduction cells include two types of cells: the first exhaust gas introduction cells; and the second exhaust gas introduction cells each of which is designed such that its cross sectional area in a direction perpendicular to the longitudinal direction of the cells is larger than that of each first exhaust gas introduction cell, and the cross sectional area of each exhaust gas emission cell in a direction perpendicular to the longitudinal direction of the cells is equal to or larger than the cross sectional area of each second exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cell.

In addition, because the exhaust gas introduction cells are arranged to fully surround the exhaust gas emission cells across cell walls, there are no other openings from which exhaust gas can flow out around each exhaust gas emission cell on the exhaust gas emission side. This structure is less likely to cause large eddying flow or the like upon emission of exhaust gas. This lowers the outflow resistance of the factor (f). Moreover, because the entire area of the cell walls can be used for filtration, PMs are likely to accumulate thinly and uniformly on the inner walls of the exhaust gas introduction cells, lowering the passage resistance of the factor (d). Thus, in the provided honeycomb filter, the pressure loss is small at the initial stage and does not easily increase until the amount of accumulated PMs is 6 to 10 g/L.

Preferably, in the honeycomb filter of the embodiments of the present invention, the first exhaust gas introduction cell has a cross sectional area of 0.5 mm² or larger in a direction perpendicular to the longitudinal directions of the cells.

If the cross sectional area of each first exhaust gas introduction cell is smaller than 0.5 mm², the opening area of the first exhaust gas introduction cell becomes too small before the amount of accumulated PM is 6 g/L, thus unfortunately increasing the pressure loss prior to the excessive capture.

In addition to the above described structure,

the honeycomb filter of the embodiments of the present invention preferably has the following structure: in the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells and the exhaust gas introduction cells are each polygonal, and a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the first exhaust gas introduction cell is longer than a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the second exhaust gas introduction cell; or one of the sides forming the cross sectional shape of the first exhaust gas introduction cell faces one exhaust gas emission cell, and none of the sides forming the cross sectional shape of the second exhaust gas introduction cell faces the sides forming the exhaust gas emission cell.

In the honeycomb filter having the above structure, the cross sectional area of each exhaust gas emission cell and the cross sectional area of each exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells are each polygonal. Owing to the above-described structure of the exhaust gas emission cells and the exhaust gas introduction cells, the honeycomb filter can not only reduce the initial pressure loss as compared with conventional honeycomb filters but also reduce the rate of increase in the pressure loss even after accumulation of a considerable amount of PMs on the cell walls. The pressure loss can be maintained small from the initial stage to a point where the amount of accumulated PMs reaches 6 to 10 g/L.

The phrase “cross sectional shape of a cell” herein refers to a shape formed by an inner cell wall of the exhaust gas emission cell, first exhaust gas introduction cell, or second exhaust gas introduction cell in the direction perpendicular to the longitudinal direction of the cell.

The phrase “cross sectional area of a cell” herein refers to an area of a cross sectional shape formed by an inner cell wall of each exhaust gas emission cell, first exhaust gas introduction cell, or second exhaust gas introduction cell in a cross section perpendicular to the longitudinal direction of the cell. The term “inner cell wall” refers to a surface on the inner side of a cell among surfaces of cell walls defining rims of cells.

Moreover, the term “side” herein refers to a segment between vertices of a polygon in the case where cross sectional shapes floored by inner cell walls of the exhaust gas emission cells, the first exhaust gas introduction cells, or the second exhaust gas introduction cells are polygons in a direction perpendicular to the longitudinal direction of the cells.

The phrase “volume of a cell” refers to a volume of a part surrounded by inner walls except for the plugged portion of the cell. The term “total volume” refers to the total sum of the volumes. The volume of the cell is calculated by multiplying the cross sectional area of the cell and the length of the cell excluding the length of the plugged portion.

Furthermore, the term “length of a side” means the length of the segment. In the case of roundly-cornered shapes with the vertex portions formed by curved lines (so-called chamfered shape), the length of a side means the length of a straight line excluding the curved line portions for the following reasons.

In the case where the vertex portions are formed by curved lines, the cell walls separating the cells are thick in the curve portions, and thus the curve portions have high passage resistance. This causes exhaust gas to preferentially flow into straight line portions, and thus the length of the straight portions needs to be controlled. Hence, it is reasonable to exclude the curve portions from consideration.

Provided that the straight portions of a polygon are hypothetically extended, and intersections of the hypothetical straight lines are given as hypothetical vertices, the length of the straight portion of the side excluding the curve portion is preferably not less than 80% the length of a hypothetical side given by connecting the hypothetical vertices. In the case of the cell having a polygonal cross sectional shape in which the sides have not less than 80% the length of the hypothetical sides, a main-channel-switching effect, which is a functional effect of the embodiments of the present invention, can be achieved by adjusting the length of the sides.

In the honeycomb filter according to the embodiments of the present invention, a side forming the cross sectional shape of a first exhaust gas introduction cell or a second exhaust gas introduction cell is considered to face an exhaust gas emission cell when the following condition is satisfied. Provided that, in the cross section perpendicular to the longitudinal direction of cells, a hypothetical perpendicular lane (hereinafter referred to as a perpendicular bisector) which bisects a side of a polygon formed by the inner cell wall of a first exhaust gas introduction cell or a second exhaust gas introduction cell is given from the side to outside the first exhaust gas introduction cell or the second exhaust gas introduction cell, the perpendicular bisector crosses a shape region defined by the inner cell wall of an exhaust gas emission cell which is adjacent to the first exhaust gas introduction cell or a second exhaust gas introduction cell across a cell wall.

In such a case, a first exhaust gas introduction cell or second exhaust gas introduction cell having a side facing an exhaust gas emission cell is considered to face the exhaust gas emission cell.

In the honeycomb filter according to the embodiments of the present invention, a side forming the cross sectional shape of an exhaust gas emission cell is considered to face a first exhaust gas introduction cell or a second exhaust gas introduction cell when the following condition is satisfied. Provided that, in the cross section perpendicular to the longitudinal direction of cells, a hypothetical perpendicular line (hereinafter referred to as a perpendicular bisector) which bisects a side of a polygon formed by the inner cell wall of an exhaust gas emission cell is given from the side to outside the exhaust gas emission cell, the perpendicular bisector crosses a shape region defined by the inner cell wall of a first exhaust gas introduction cell or a second exhaust gas introduction cell which is adjacent to the exhaust gas emission cell across a cell wall.

In such a case, an exhaust gas emission cell having a side facing a first exhaust gas introduction cell or a second exhaust gas introduction cell is considered to face the first exhaust gas Introduction cell or the second exhaust gas introduction cell.

Moreover, in the honeycomb filter according to the embodiments of the present invention, a side forming a first exhaust gas introduction cell is considered to face a second exhaust gas introduction cell when the following condition is satisfied. Provided that, in the cross section perpendicular to the longitudinal direction of cells, a hypothetical perpendicular line (hereinafter referred to as a perpendicular bisector) which bisects a side of a polygon formed by the inner cell wall of a first exhaust gas introduction cell is given from the side to outside the first exhaust gas introduction cell, the perpendicular bisector crosses a shape region defined by the inner cell wall of a second exhaust gas introduction cell which is adjacent to the first exhaust gas introduction cell across a cell wall.

In such a case, a first exhaust gas introduction call having a side facing a second exhaust gas introduction cell is considered to face the second exhaust gas introduction cell.

Furthermore, in the honeycomb filter according to the embodiments of the present invention, a side forming a second exhaust gas introduction cell is considered to face a first exhaust gas introduction cell when the following condition is satisfied. Provided that, in the cross section perpendicular to the longitudinal direction of cells, a hypothetical perpendicular line (hereinafter referred to as a perpendicular bisector) which bisects a side of a polygon formed by the inner cell wall of a second exhaust gas introduction cell is given from the side to outside the second exhaust gas introduction cell, the perpendicular bisector crosses a shape region defined by the inner cell wall of a first exhaust gas introduction cell which is adjacent to the second exhaust gas introduction cell across a cell wall.

In such a case, the second exhaust gas introduction cell having a side facing a first exhaust gas introduction cell is considered to face the first exhaust gas introduction cell.

In the honeycomb filter of the embodiments of the present invention, the thickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal direction of cells, provided that a hypothetical straight line is given which connects geometric centers of gravity of cross sectional figures defined by the inner cell walls of two cells, the length of a segment of the straight line overlapping the cell wall area is defined as the thickness of the cell wall. Although cells are void, the centers of gravity herein refer to geometric centers of gravity of cross sectional figures defined by inner cell walls. Thus, the center of gravity can be defined even for cross sectional figures of void seem as cells.

The word “adjacent” herein is equivalent to the word “adjacent” in Japanese. The word “adjacent” is used not only for a case where exhaust gas introduction cells are arranged to face exhaust gas emission cells across porous cell walls, but also for a case where exhaust gas introduction cells are not facing but arranged diagonally to exhaust gas emission cells across porous cell walls. In Japanese, the expression “lattices are diagonally adjacent to each other” is accepted as an exemplary expression using “adjacent”.

The case where exhaust gas introduction cells and exhaust gas emission cells each have a polygonal cross sectional shape and are arranged to face each other across porous cell walls is specifically illustrated in FIGS. 2A to 2C, 4A, and 4B. In FIGS. 2A to 2C, 4A, and 4B, second exhaust gas introduction cells 14 face exhaust gas emission cells 11 across porous cell walls 13. In a case where exhaust gas introduction cells and exhaust gas emission cells each have a round or elliptical cross sectional shape and a single porous cell wall is formed by curves of the cross sectional shapes of an exhaust gas introduction cell and an exhaust gas emission cell (a case where a curve of the inner wall of an exhaust gas emission cell and a curve of the inner wall of an exhaust gas introduction cell form the front side and the rear side of a single cell wall in a three-dimensional view), the exhaust gas introduction cell and the exhaust gas emission cell are considered to be arranged to face each other across a porous cell wall according to the case where exhaust gas introduction cells and exhaust gas emission cells each have a polygonal cross sectional shape. Specifically, such a case is illustrated in FIG. 14. In FIG. 14, second exhaust gas introduction cells 54 are considered to face exhaust gas emission cells 51 across porous cell walls 53.

The case where exhaust gas introduction cells and exhaust gas emission cells each have a polygonal cross sectional shape and are not arranged to face each other but arranged diagonally across porous cell walls is specifically illustrated in FIGS. 12A to 12C. In FIGS. 12A to 12C, second exhaust gas introduction cells 44 and exhaust gas emission cells 41 do not face each other and are arranged diagonally across porous cell walls 43.

In a case where exhaust gas introduction cells and exhaust gas emission cells each have a shape formed by curved lines, except for round or elliptical shape, an intersection of two curved lines is considered to be a vertex and a curved line between two vertices is considered to be a side. Here, a side (curved line) of an exhaust gas emission cell or exhaust gas introduction cell is considered to face an exhaust gas introduction cell or exhaust gas emission cell when the following condition is satisfied. Provided that a hypothetical perpendicular line which bisects a segment between vertices at the both ends of a side (curved line) forming the cross sectional shape of an exhaust gas emission cell or an exhaust gas introduction cell (A) is given from the side toward the outside the exhaust gas emission cell or the exhaust gas introduction cell (A), the perpendicular bisector crosses a shape region defined by the inner cell wall of an exhaust gas introduction cell or an exhaust gas emission cell (B) that is closest to the cell (A) across a cell wall. Moreover, the exhaust gas emission cell or the exhaust gas introduction cell (A) is described to face the exhaust gas introduction cell or the exhaust gas emission cell (B). In the case of the vertex portions formed by curved lines (so-called chamfered shape), the curved lines are extended, and the hypothetical intersection of the extended lines is considered to be a vertex.

In a case where exhaust gas introduction cells and exhaust gas emission cells each have a shape formed by curved lines, except for round or elliptical shape, a case where exhaust gas introduction cells and exhaust gas emission cells do not face each other and sure arranged diagonally across porous cell walls is specifically illustrated in FIG. 15. In FIG. 15, second exhaust gas introduction cells 64 and exhaust gas emission cells 61 do not face each other and are arranged diagonally across porous cell walls 63.

The sentence “each of the exhaust gas emission cells is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls” can encompass “each exhaust gas emission cell is enclosed all around by the adjacent exhaust gas introduction cells across the porous cell walls” in the present application.

Here, the phrase “arranged diagonally” refers to the arrangement in which exhaust gas introduction cells and exhaust gas emission cells do not face each other and satisfy the following condition. In a cross section perpendicular to the longitudinal direction of an exhaust gas emission cell, a hypothetical segment between the geometrical center of gravity and a vertex (in the case of chamfered vertex portions, sides (straight or curved lines) forming the cross sectional figure are hypothetically extended and an intersection of the extended lines is considered to be a vertex) of a cross sectional figure formed by the inner cell wall of the exhaust gas emission cell is provided. In a cross section perpendicular to the longitudinal direction of an exhaust gas introduction cell, a hypothetical segment between the geometrical center of gravity and a vertex (in the case of chamfered vertex portion, sides (straight or curved lines) forming the cross sectional figure are hypothetically extended and an intersection of the extended lines is considered to be a vertex) of a cross sectional figure formed by the inner cell wall of the exhaust gas introduction cell is provided. Here, the provided hypothetical segments are parallel with each other or overlapped with each other. It is to be noted that, if a pair of hypothetical lines among plural hypothetical lines is parallel or overlapped with each other, the other hypothetical lines may be across with each other at a predetermined angle (e.g., 90°).

In the description of the term “adjacent”, the term “exhaust gas introduction cell” refers to both the first exhaust gas introduction cell and the second exhaust gas introduction cell.

Next, a side which faces a cell and the thickness of a cell wall separating two cells are explained below based on figures.

FIG. 1 is an enlarged end face view illustrating an enlarged image of a part of an end face of the honeycomb filter according to one embodiment of the present invention. FIG. 1 illustrates exhaust gas emission cells 11, and first exhaust gas introduction cells 12 and second exhaust gas introduction cells 14 surrounding the exhaust gas emission cells 11.

A side forming the cross sectional shape of a first exhaust gas introduction cell 12 or a second exhaust gas introduction cell 14 is considered to face an exhaust gas emission cell 11 when the following condition is satisfied. Provided that, in the cross section perpendicular to the longitudinal direction of the cells shows in FIG. 1, a hypothetical perpendicular line (hereinafter referred to as a perpendicular bisector) which bisects a side 12 a of a polygon formed by the inner cell wall of a first exhaust gas introduction cell 12 or a side 14 a of a polygon formed by the inner cell wall of a second exhaust gas introduction cell 14 is given from the side to outside the first exhaust gas introduction cell 12 or the second exhaust gas introduction cell 14, the perpendicular bisector A or the perpendicular bisector B crosses a shape region (side 11 a, side 11 b) defined by the inner cell wall of an exhaust gas emission cell 11 which is adjacent to the side 12 a of the first exhaust gas introduction cell 12 or the side 14 a of the second exhaust gas introduction cell 14 across a cell wall as shown in FIG. 1.

The reason why the crossing of the bisector is set as a condition for the facing in the embodiments of the present invention is that the passage resistance caused upon allowing exhaust gas to pass through at or around the center of the side in the length direction, i.e. at or around the center of the cell wall separating the exhaust gas introduction cell and the exhaust gas emission cell, represents a pressure loss caused upon allowing exhaust gas to pass through the entire wall.

According to the embodiments of the present invention, in the case where the cross sectional shape defined by the inner cell wall of each exhaust gas emission cell, first exhaust gas introduction cell, or second exhaust gas introduction cell is a polygon, and the vertex portions of the polygon are chamfered, i.e. formed by curved lines in the cross section perpendicular to the longitudinal direction of the cells, the bisectors of respective sides are bisectors of segments excluding the curved lines.

Moreover, in the case where the vertex portions are roundly-cornered, i.e. formed by curved lines, the curved lines are not included in the sides. In the case where the vertex portions are chamfered in the cross sectional shape, the sides forming the cross sectional shape are hypothetically extended, and intersections of the extended sides are considered as hypothetical vertices. Hence, the cross sectional shape is treated as a polygon.

This is based on the following intention. For manufacturing a honeycomb filter including cells having polygonal cross sectional shapes in the cross section perpendicular to the longitudinal direction of the cells by extrusion molding, the vertex portions of the polygonal cross sectional shapes may be formed by curved lines to prevent concentration of stress at the vertex portions. Such cross sections in which the vertex portions are formed by curved lines are considered as polygons.

The thickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal direction of the cells shown in FIG. 1, provided that a hypothetical line Z₁₄ is given which connects geometric centers of gravity of the cross sectional figures defined by the inner cell walls of the two cells (the center of gravity of the exhaust gas emission cell 11 is O₁₁, and the center of gravity of the second exhaust gas introduction cells is O₁₄ in FIG. 1), the length D of the segment of the line Z₁₄ overlapping the cell wall area is defined as the thickness of the cell wall. Although cells are voids, the centers of gravity herein refer to geometric centers of gravity of cross sectional figures defined by inner cell walls. Thus, the center of gravity can be defined even for cross sectional figures of voids such as cells.

The thickness of the cell wall is defined as above for the following reasons. The resistance caused upon allowing gas to pass through the cell walls is the highest at a portion of the cell wall where the gas passes through at the highest flow rate, and the passage resistance at the portion may represent the passage resistance of the cell wall. The flow rate of gas in the longitudinal direction of the honeycomb filter is the highest at positions corresponding to the geometric centers of gravity of the cross sectional shapes defined by the inner cell walls. The flow rate concentrically decreases from the center of gravity toward the circumference of the cross sectional shape of the cells. Thus, the flow rate of gas passing through a cell wall is the highest at an intersection of the cell wall and a line connecting the center of gravity of an exhaust gas introduction cell and the center of gravity of an exhaust gas emission cell. In view of the pressure loss as described above, it is reasonable to define a length D of the segment of a portion where a straight line connecting the centers of gravity overlaps the cell wall area as the thickness of a cell wall.

According to the embodiments of the present invention, electron microscope pictures are used for measurement of the length of sides and the thickness of cell walls, and identification of the cross sectional shapes of cells. The electron microscope pictures are taken with an electron microscope (FE-SEM: high resolution field emission scanning electron microscope S-4800, manufactured by Hitachi High-Technologies Corporation).

The electron microscope pictures are preferably those taken by the electron microscope at a magnification of 30×. This is because, at a magnification of 30×, irregularities due to particles or pores on the surface (inner wall) of cell walls defining the rims of cells do not disturb identification of the cross sectional shapes of the cells, measurement of the lengths of sides, thicknesses of cell walls, and cross sectional area of the cells. Also, at a magnification of 30×, the cross sectional shapes of the cells can be identified, and the lengths of sides, thicknesses of cell walls, and cross sectional area of the cells can be measured.

In other words, the lengths of the respective sides of the cell are measured using the scale of the electron microscope photographs based on the above definitions of the length of the cells and the thickness of the cell walls. The cross sectional area is arithmetically determined based on the obtained values including the length of the cells. If arithmetic calculation of the cross sectional area is complicated, the cross sectional area can also be determined by cutting a square piece (a square with a side having a scale length) corresponding to a unit area out of the electron microscope photograph, and weighing the cut-out piece, while separately cutting the cross section of the cell out along the cross sectional shape of the cell (along curved lines in the case of a polygonal cross section with the vertex portions formed by curved lines), and weighing the cut-out piece, and then calculating the cross sectional area of the cell based on the weight ratio.

For example, the cross sectional shapes defined by the inner walls of the exhaust gas emission cells and the second exhaust gas introduction cells may be octagons having the same cross sectional area from one another, and the cross sectional shapes defined by the respective inner walls of the first exhaust gas introduction cells may be square (although the vertex portions have so-called roundly-cornered shape (i.e. formed by curved lines), the cross sectional shapes are each considered as a square having four sides and four vertices at four intersections of straight lines extended from the four sides forming the cross sectional shape in the embodiments of the present invention). In the above case, a square (corresponding to a unit area) having each side in a length corresponding to 500 μm in the photograph is cut out of the photograph, and the cut-out piece is weighed. Next, the octagon and the square are cut out of the photograph (the four vertex portions formed by curved lines in the square are cut along the curved lines), and the cut-out pieces are weighed. The cross sectional area is calculated based on the weight ratio between the out-out piece and the 500 μm scale square. In the case of measuring only the cross sectional area ratio of the cells, the area ratio can be obtained directly from the weight ratio between the octagon and the square.

According to the embodiments of the present invention, the measurement of the lengths of the cells, the thicknesses of the cell walls, and the cross sectional areas can be changed from the above manual measurements to an electronic measurement by scanning the electron microscope photograph as image data, or using the image data directly output from the electron microscope and entering the scale of the photograph. The manual measurement and the electronic measurement are both based on the scale of the electron microscope image, and are in accordance with the same principle. Thus, surely no discrepancies are found between the measurement results of the respective measurements.

For example, the electronic measurement may be performed by using an image analysis and grain size distribution measurement software (Mac-View (Version 3.5), produced by Mountech Co. Ltd.). This software measures a cross sectional area by scanning an electron microscope photograph as image data, or using the image data directly output from the electron microscope, entering the scale of the photograph, and specifying the area along the inner wall of the cell. Moreover, the distance between any two points in the image can be measured based on the scale of the electron microscope photograph.

A photograph of the cross section of the cells is taken with the electron microscope by cutting a filter in a direction perpendicular to the longitudinal direction of the cells to prepare a 1 cm×1 cm×1 cm sample including the cut face and ultrasonic cleaning the cut section of the sample, or coating a filter with resin and cutting the coated filter in a direction perpendicular to the longitudinal direction of the cells, and then taking an electron microscope photograph of the sample. The resin coating does not affect measurement of the lengths of the sides of the cells and the thicknesses of the cell walls.

A convex square according to the embodiments of the present invention is a shape formed by four outwardly curved lines of the same length. The square looms as if its sides bulge from the geometric center of gravity to the outside. A concave square is a shape formed by four inwardly curved lines of the same length. The square looks as if its sides concave toward the geometric center of gravity.

In the cross section perpendicular to the longitudinal direction of first cells forming the honeycomb filter according to the embodiments of the present invention.

the first exhaust gas introduction cells, the second exhaust gas introduction cells, and the exhaust gas emission cells each have a uniform cross sectional shape thoroughly from the exhaust gas introduction end to the exhaust gas emission end except for the plugged portion. Namely, taking the first exhaust gas introduction cell as an example, in a cross sectional view perpendicular to the longitudinal direction, the cross sectional shape defined by the inner wall thereof has the same shape at any part from the exhaust gas introduction end to the exhaust gas emission end except for the plugged portion. The same shape means a congruent shape, and excludes similar shapes. Namely, a similar shape means a different shape. The explanation for the first exhaust gas introduction cell also applies to the second exhaust gas introduction cell and the exhaust gas emission cell. The plugged portions are excluded because the cross sectional shape defined by the inner cell wall does not physically exist at the plugged portions due to the presence of the plugs.

The honeycomb filter baaing the above structure can comprehensively reduce the pressure loss thoroughly from the initial stage to the stage after accumulation of PMs in close to the limit amount, as compared to conventional honeycomb filters.

In consideration of the above-described pressure loss broken down to respective resistance components, the flow-through resistance and the outflow resistance need to be reduced for reducing the initial pressure loss. Thus, the cross sectional area of the exhaust gas emission cells needs to be equal to or relatively larger than the cross sectional area of the exhaust gas introduction cells for suppressing the rapid expansion. Wide and thin accumulation of PMs is necessary for reducing the transitional pressure loss. Thus, the cross sectional area of the exhaust gas introduction cells needs to be relatively larger than the cross sectional area of the exhaust gas emission cells.

It has been considered impossible to reduce both of the transitional pressure loss and the initial pressure loss. The inventors of the present invention further studied and completed the embodiments of the present invention described below.

That is, exhaust gas is preferentially introduced firstly into the first exhaust gas introduction cells when the honeycomb filter has the following structures: two kinds of the exhaust gas introduction cells including the exhaust gas introduction cells each having a larger cross sectional area (second exhaust gas introduction cells) and the exhaust gas introduction cells each having a smaller cross sectional area (first exhaust gas introduction cells) are employed as the exhaust gas introduction cells; each exhaust gas emission cell has an equal or larger cross sectional area than each second exhaust gas introduction cell; each exhaust gas emission cell is fully surrounded by the two kinds of the exhaust gas introduction cells; and the length of the inner wall separating the first exhaust gas introduction cell and the exhaust gas emission cell is relatively longer than the length of the inner wall separating the second exhaust gas introduction cell and the exhaust gas emission cell, or the thickness of the wall separating the first exhaust gas introduction cell and the exhaust gas emission cell is relatively smaller than the thickness of the wall separating the second exhaust gas introduction cell and the exhaust gas emission cell.

The wall separating the first exhaust gas introduction cell and the exhaust gas emission cell has a larger passage area (in the case of polygonal cells, sides are long in the cross sectional shape) or the thickness of the wall is smaller. Exhaust gas can thus pass through such an advantageous wall so that the passage resistance of the factor (c) can be reduced. Also, the flow-through resistance of the factor (e) can be reduced as the cross sectional area of the exhaust gas emission cells is relatively larger than the cross sectional area of the first exhaust gas introduction cells. Namely, both of the passage resistance of the factor (c) and the flow-through resistance of the factor (e) can be reduced, and thereby the initial pressure loss can be reduced. After accumulation of a certain amount of PMs, because the cross sectional area of the first exhaust gas introduction cells is smaller than the cross sectional area of the second exhaust gas introduction cells, an increase in the passage resistance occurs earlier in a layer of the accumulated PMs in the first exhaust gas introduction cells. This leads to “switching” of the main flow channel of exhaust gas in a manner that a larger amount of the exhaust gas is naturally (i.e. autonomously) introduced to the second exhaust gas introduction cells. Consequently, PMs widely and thinly accumulate in the second exhaust gas introduction cells each having a large cross sectional area. Hence, both of the flow-through resistance of the factor (b) and the passage resistance of the factor (d) can be reduced so that the transitional pressure loss can be reduced even after accumulation of PMs.

As described above, the embodiments of the present invention have achieved a surprising effect, which has been considered impossible, of reducing both of the transitional pressure loss and the initial pressure loss by the autonomous switching of the main flow channel.

The aforementioned effect of reducing both of the initial pressure loss and the transitional pressure loss by “switching” of the main flow channel to which a larger amount of exhaust gas is introduced is exerted only when all the aforementioned features work integrally. Such structures or effects are not disclosed in any publicly known document.

The aforementioned international application: WO 2004/024294 discloses a honeycomb filter including exhaust gas introduction cells 102 each having an octagonal cross sectional shape and exhaust gas emission cells 101 each having a rectangular cross sectional shape as shown in FIG. 17B. WO 2004/024294 discloses that an increase in the cross sectional area of the exhaust gas introduction cells 102 enables wide and thin accumulation of PMs, and thus the transitional pressure loss can be reduced. However, for achieving the present invention in view of WO 2004/024294, some of the exhaust gas emission cells 101 each having a smaller cross sectional area need to be changed to the exhaust gas introduction cells 102, and some of the exhaust gas introduction cells 102 each having a larger cross sectional area need to be changed to the exhaust gas emission cells 101. Such changes deny the inventive concept of the invention of WO 2004/024294, i.e. increasing the cross sectional area of the exhaust gas introduction cells 102. Thus, the present invention cannot be achieved in view of WO 2004/024294 as the closest background art document.

Also, as explained earlier based on FIGS. 18A and 18B, U.S. Pat. No. 4,417,908 discloses a honeycomb filter which can reduce the transitional pressure loss by increasing the number of exhaust gas introduction cells having the same cross sectional area to increase the total area of the exhaust gas introduction cells so that PMs are allowed to widely and thinly accumulate.

However, for achieving the present invention in view of U.S. Pat. No. 4,417,908, some of the exhaust gas introduction cells need to be changed to cells having a smaller cross sectional area. Such a change reduces the cress sectional area of the exhaust gas introduction cells, and thus denies the inventive concept of U.S. Pat. No. 4,417,908. Hence, the present invention is not achieved in view of U.S. Pat. No. 4,417,908 as the closest background art document.

Those background art documents deny the present invention, and thus the present invention cannot be achieved in view of the background arts.

The following will explain the details of the effects of the embodiments of the present invention by exemplifying an embodiment.

FIGS. 2A to 2C are each an enlarged end face view illustrating an enlarged image of a part of an end face of the honeycomb filter according to one embodiment of the present invention.

As shown in FIG. 2A, in a honeycomb filter 10, each exhaust gas emission cell 11 having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side is adjacently surrounded fully by first exhaust gas introduction cells 12 and second exhaust gas introduction cells 14 each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side across porous cell walls 13.

In the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cell 11 has an octagonal cross section that is the same as or similar to that of the exhaust gas introduction cell 102 shown in FIG. 17B, the first exhaust gas introduction cell 12 has a square cross section, and the second exhaust gas introduction cell 14 has an octagonal square section that is the same as that of the exhaust gas emission cell 11. The second exhaust gas introduction cell 14 has a larger cross sectional area than the first exhaust gas introduction cell 12, and the cross sectional area is the same as the exhaust gas emission cell 11. That is, the second exhaust gas introduction cell 14 has the same cross sectional area as the exhaust gas emission cell 11, and the exhaust gas emission cell 11 has a larger cross sectional area than the first exhaust gas introduction cell 12. Thus, the resistance caused by flowing of exhaust gas through the exhaust gas emission cells 11 and the resistance caused by outflow of exhaust gas to outside the filter are reduced to low levels, and thereby the pressure loss can be reduced to a low level.

Moreover, the side 12 a facing the exhaust gas emission cell 11 among the sides forming the cress sectional shape of the first exhaust gas introduction cell 12 is longer than the side 14 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14.

Exhaust gas flowing toward a honeycomb filter 20 flows into the first exhaust gas introduction cells 12 each having an open end at the exhaust gas introduction side and the second exhaust gas introduction cells 14 each having an open end at the exhaust gas introduction side. The exhaust gas flows in the filter in order from a part allowing easier flow and then evenly flows in the entire filter. In the honeycomb filter according to the embodiments of the present invention, the length (Ls) of the side 12 a of the exhaust gas emission cell 11 is larger than the length (Lo) of the side 14 a of the second exhaust gas introduction cell 14. Thus, the surface area of a cell wall 13 a separating the exhaust gas emission cell 11 and the first exhaust gas introduction cell 12 is larger than the surface area of a cell wall 13 b separating the exhaust gas emission cell 11 and the second exhaust gas introduction cell 14, leading to easier exhaust gas passage through the cell wall 13 a. Consequently, PMs accumulate on the cell walls 13 a at an initial stage.

As described above, both of the flow-through resistance of the exhaust gas emission cells and the outflow resistance upon emission of exhaust gas from the honeycomb filter can be reduced. Thus, the initial pressure loss before accumulation of PMs can be reduced.

The relation between the length of a side forming a cell and the surface area mentioned above is attributed to the following reasons.

The surface area of the cell wall 13 a separating the exhaust gets emission cell 11 and the first exhaust gas introduction cell 12 corresponds to the surface area of the inner wall of the first exhaust gas introduction cell 12. The surface area of the inner wall of the first exhaust gas introduction cell 12 is expressed as Ls×Le, where Le represents an effective length of the filter excluding the length of the plugged portions at the introduction side from the exhaust gas introduction end and at the emission side from the exhaust gas emission end (see FIG. 4B). Similarly, the surface area of the cell wall 13 b separating the exhaust gas emission cell 11 and the second exhaust gas introduction cell 14 corresponds to the surface area of the inner wall of the second exhaust gas introduction cell 14. The surface area of the inner wall of the second exhaust gas introduction cell 14 is expressed as Lo×Le, where Le represents an effective length of the filter excluding the length of the plugged portions at the introduction side from the exhaust gas introduction end and at the emission side from the exhaust gas emission end. The effective length of the filter is defined as a length reassured from the tip of a plug in FIG. 4B.

Thus, if the length (Ls) of the side 12 a is longer than the length (Lo) of the side 14 a, the surface area value of Ls×Le is relatively larger than the surface area value of Lo×Le. Namely, the length of the side is equivalent to the surface area. Thus, if the length (Ls) of the side 12 a of the exhaust gas emission cell 11 is longer than the length (Lo) of the side 14 a of the second exhaust gas introduction cell 14, the surface area of the cell wall 13 a separating the exhaust gas emission cell 11 and the first exhaust gas introduction cell 12 is larger than the surface area of the cell wall 13 b separating the exhaust gas emission cell 11 and the second exhaust gas introduction cell 14.

In FIGS. 2A to 2C, insertions relating to the effects are only partially illustrated. The same is true to FIG. 6.

Next, as shown in FIG. 2B, when a certain amount of PMs accumulate on the cell walls 13 a corresponding to the inner walls of the first exhaust gas introduction cells 12, an accumulated layer of the PMs gets thick due to the small cross sectional area of the first exhaust gas introduction cells 12. Consequently, resistance due to the accumulation of PMs increases to mate the passage of exhaust gas through the cell walls 13 a difficult. Under such conditions, exhaust gas turns to pass through the cell walls 13 b separating the exhaust gas emission cells 11 and the second exhaust gas introduction cells 14 (switching of the main channel). Then, PMs also accumulate on the surfaces of the cell walls 13 b corresponding to the surfaces of the inner walls of the second exhaust gas introduction cells 14.

Subsequently, because exhaust gas can considerably freely pass through the cell walls, exhaust gas passes through inside the cell walls 13 c separating the first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 to flow into the exhaust gas emission cells 11 as shown in FIG. 2C. In this situation, exhaust gas enters the cell walls 13 c from the first exhaust gas introduction cells 12 as well as from the second exhaust gas introduction cells 14.

As described above, PMs accumulate on the entire surfaces of the cell walls 13 a and 13 c around the first exhaust gas introduction cells 12 corresponding the inner walls of the first exhaust gas introduction cells 12, and gradually accumulate rather more widely and thinly in a larger amount on the entire surfaces of the cell walls 13 b and 13 c around the second exhaust gas introduction cells 14 corresponding to the inner walls of the second exhaust gas introduction cells 14. The first exhaust gas introduction cells 12 each have a smaller cross sectional area than each second exhaust gas introduction cell 14. Thus, in the first exhaust gas introduction cells 12, PMs accumulate in a thick layer where the passage resistance is high. For this reason, the introduced exhaust gas flows more easily into the second exhaust gas introduction cells 14 than into the first exhaust gas introduction cells 12 at an early stage (aforementioned switching of the main channel of exhaust gas), causing the aforementioned shift of the PM accumulation. Consequently, PMs accumulate more on the entire surfaces of the cell walls 13 b and 13 c around the second exhaust gas introduction cells 14 corresponding to the inner walls of the second exhaust gas introduction cells 14 rather than the surfaces of the cell walls 13 a and 13 c around the first exhaust gas introduction cells 12 corresponding to the inner walls of the first exhaust gas introduction cells 12. It is thus possible to make use of the entire surfaces of the cell walls 13 b and 13 c around the second exhaust gas introduction cells 14 corresponding to the inner walls of the second exhaust gas introduction cells 14 for accumulation of PMs earlier. Since the surface area of the cell walls 13 b and 13 c around the second exhaust gas introduction cells 14 corresponding to the inner walls of the second exhaust gas introduction cells 14 is larger than the surface area of the cell walls 13 a and 13 c around the first exhaust gas introduction cells 12 corresponding to the inner walls of the first exhaust gas introduction cells 12, when PMs accumulate on the entire peripheries of the cell walls 13 b and 13 c surrounding the second exhaust gas introduction cells 14, the accumulated layer is thin. Thus, the pressure loss due to exhaust gas increases at a low rate even after accumulation of PMs. Hence, a surprisingly excellent effect of maintaining the pressure loss at a low level can be achieved regardless of an increase in the amount of accumulated PMs, until the amount of accumulated PMs reaches 6 to 10 g/L.

As a result, vehicles carrying the honeycomb filter according to the embodiments of the present invention do not cause a disadvantageous phenomenon for driving that are derived from an increase in the pressure loss until the amount of accumulated PMs reaches 6 to 10 g/L, and also have good fuel economy.

The honeycomb filter of the embodiments or the present invention preferably has the following structure. In the cross section perpendicular to the longitudinal direction of an exhaust gas emission cell and a first exhaust gas introduction cell which have polygonal cross sectional shapes and are adjacent to each other, a side, among the sides forming the cross sectional shape of the exhaust gas emission cell, which is adjacent to the first exhaust gas introduction cell across the cell wall in a manner facing the first exhaust gas introduction cell is parallel to a side, among the sides forming the cross sectional shape of the first exhaust gas introduction cell, which is adjacent to the exhaust gas emission cell across the cell wall in a manner facing the exhaust gas emission cell.

This indicates that the thickness is uniform at any part of the walls separating the exhaust gas emission cells and the first exhaust gas introduction cells. Thus, it is possible to achieve high fracture strength of the honeycomb filter, easy passage of exhaust gas, and uniform accumulation of PMs, so that the pressure loss is reduced.

In the case where the vertex portions of the polygonal cross section are formed by curved lines, the curve portions are not considered as sides because naturally such portions do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinal direction of the cells, straight portions considered as sides are hypothetically extended, and intersections of the hypothetical straight lines are given as hypothetical vertices, the length of each side of the cross sectional shape excluding the curve portion is preferably not less than 80% the length of a hypothetical side of a polygon that is formed by connecting the hypothetical vertices. To put it the other way around, the length of the portion not considered as a side is preferably less than 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if the lengths of the sides are not less than 80% the respective lengths of the hypothetical sides, the main-channel-switching effect, which is an effect of the embodiments of the present invention, can be achieved by controlling the length of the sides.

The honeycomb filter of the embodiments of the present invention preferably has the following structure. In the cross section perpendicular to the longitudinal direction of an exhaust gas emission cell and a second exhaust gas introduction cell which is adjacent to the exhaust gas emission cell across a cell wall, in the case where the cells have polygonal cross sections, a side, among the sides forming the cross sectional shape of the exhaust gas emission cell, which is adjacent to the second exhaust gas introduction cell across the cell wall in a manner facing the second exhaust gas introduction cell is parallel to a side, among the sides forming the cross sectional shape of the second exhaust gas introduction cell, which is adjacent to the exhaust gas emission cell across the cell wall in a manner facing the exhaust gas emission cell.

This indicates that the thickness is uniform at any part of the walls separating the exhaust gas emission cells and the respective second exhaust gas introduction cells. Thus, it is possible to achieve high fracture strength of the honeycomb filter, easy passage of exhaust gas, and uniform accumulation of PMs so that the pressure loss is reduced.

Meanwhile, in the case where the vertex portions of the polygonal cross sectional shape are formed by curved lines, the curve portions are not considered as sides because naturally such portions do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinal direction of the cells, straight portions considered as sides are hypothetically extended, and intersections of the hypothetical straight lines are given as hypothetical vertices, the length of each side of the cross sectional shape excluding the curve portion is preferably not less than 80% the length of a hypothetical side of a polygon that is formed by connecting the hypothetical vertices. To put it the other way around, the length of the portion not considered as a side is preferably less than 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if the lengths of the sides are not less than 80% the respective lengths of the hypothetical sides, the main-channel-switching effect, which is an effect of the embodiments of the present invention, can be achieved by controlling the length of the sides.

The honeycomb filter of the embodiments of the present invention preferably has the following structure. In the cross section perpendicular to the longitudinal direction of a first exhaust gas introduction cell and a second exhaust gas introduction cell which is adjacent to the first exhaust gas introduction cell across a cell wall, in the case where the cells have polygonal cross sectional shapes, a side, among the sides forming the cross sectional shape of the first exhaust gas introduction cell, which is adjacent to the second exhaust gas introduction cell across the cell wall in a manner facing the second exhaust gas introduction cell, is parallel to a side, among the sides forming the cross sectional shape of the second exhaust gas introduction cell, which is adjacent to the first exhaust gas introduction cell across the cell wall in a manner facing the first exhaust gas introduction cell.

This indicates that the thickness is uniform at any part of the walls separating the first exhaust gas introduction cells and the second exhaust gas introduction cells. Thus, it is possible to achieve high fracture strength of the honeycomb filter, easy exhaust gas passage through the walls from the second exhaust gas introduction cells to the exhaust gas emission cells, and wide, thin and uniform accumulation of PMs on the inner cell walls of the second exhaust gas introduction cells, so that low pressure loss can be achieved after accumulation of PMs.

Meanwhile, in the case where the vertex portions of the polygonal cross section are formed by curved lines, the curve portions are not considered as sides because naturally such portions do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinal direction of the cells, straight portions considered as sides are hypothetically extended, and intersections of the hypothetical straight lines are given as hypothetical vertices, the length of each side of the cross sectional shape excluding the curve portion is preferably not less than 80% the length of a hypothetical side of a polygon that is formed by connecting the hypothetical vertices. To put it the other way around, the length of the portion not considered as a side is preferably less than 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if the lengths of the sides are not less than 80% the respective lengths of the hypothetical sides, the main-channel-switching effect, which is an effect of the embodiments of the present invention, can be achieved by controlling the length of the sides.

The honeycomb filter according to the embodiments of the present invention preferably has the following structure. In the case where the exhaust gas emission cell, the first exhaust gas introduction cell, and the second exhaust gas introduction cell, which are adjacent to one another across cell walls, each have a polygonal cross sectional shape in the cross section perpendicular to the longitudinal direction of the cells,

(a) a side, among the sides forming the cross sectional shape of the exhaust gas emission cell, which is adjacent to the first exhaust gas introduction cell across the cell wall in a manner facing the first exhaust gas introduction cell, is parallel to a side, among the sides forming the cross sectional shape of the first exhaust gas introduction cell, which is adjacent to the exhaust gas emission cell across the cell wall in a manner facing the exhaust gas emission cell,

(b) a side, among the sides forming the cross sectional shape of the exhaust gas emission cell, which is adjacent to the second exhaust gas introduction cell across the cell wall in a manner facing the second exhaust gas introduction cell, is parallel to a side, among the sides forming the cross sectional shape of the second exhaust gas introduction cell, which is adjacent to the exhaust gas emission cell across the cell wall in a manner facing the exhaust gas emission cell, and

(c) a side, among the sides forming the cross sectional shape of the first exhaust gas introduction cell, which is adjacent to the second exhaust gas introduction cell across the cell wall in a manner facing the second exhaust gas introduction cell, is parallel to a side, among the sides forming the cross sectional shape of the second exhaust gas introduction cell, which is adjacent to the first exhaust gas introduction cell across the cell wall in a manner facing the first exhaust gas introduction cell.

Moreover, in the case where the first exhaust gas introduction cell, the second exhaust gas introduction cell, and the exhaust gas emission call such have a polygonal cross sectional shape in the honeycomb filter according to the embodiments of the present invention, the distance between the parallel sides in the above condition (a), the distance between the parallel sides in the above condition (b), and the distance between the parallel sides in the above condition (c) are preferably the same in the structure simultaneously satisfying the conditions (a), (b), and (c). Here, the distance between the sides is defined as follows: a hypothetical perpendicular line is given from an arbitrary point P in one side to a point Q in the other side, and the distance between the point P and the point Q is defined as the distance between the parallel sides.

The honeycomb filter having the above structure has the highest fracture strength, has excellent thermal shock resistance upon regeneration, can best reduce the pressure loss, regardless of the presence or absence of accumulated PMs, and can avoid thermal shock damage that occurs upon regeneration of PMs.

Meanwhile, in the case where the vertex portions of the polygonal cross section are formed by curved lines, the curve portions are not considered as sides because naturally such portions do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinal direction of the cells, straight portions considered as sides are hypothetically extended, and intersections of the hypothetical straight lines are given as hypothetical vertices, the length of each side of the cross sectional shape excluding the carte portion is preferably not less than 80% the length of a hypothetical side of a polygon that is formed by connecting the hypothetical vertices. To put it the other way around, the length of the portion not considered as a side is preferably less than 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if the lengths of the sides are not less than 80% the respective lengths of the hypothetical sides, the main-channel-switching effect, which is an effect of the embodiments of the present invention, can be achieved by controlling the length of the sides.

The honeycomb filter of the embodiments of the present invention is preferably used to purify PMs in exhaust gas discharged from internal combustion engines of automobiles. The honeycomb filter can reduce the initial pressure loss before accumulation of PMs and the transitional pressure loss caused by accumulation of PMs in the filter until the amount of accumulated PMs reaches 6 to 10 g/L, and thereby the fuel economy of the engine can be enhanced.

The honeycomb filter of the embodiments of the present invention is most suitably used in automobiles whose internal combustion engines are diesel engines. The amount of PMs (soot) discharged from a diesel engine is larger than that from a gasoline engine. Thus, a demand for reducing the transitional pressure loss caused by accumulation of PMs (soot) in the filter is higher for diesel engines than for gasoline engines.

In the case of using the honeycomb filter of the embodiments of the present invention to purify PMs in exhaust gas discharged from internal combustion engines of automobiles, the honeycomb filter of the embodiments of the present invention is fixed inside an exhaust tube via a holding material.

In the honeycomb filter according to the embodiments of the present invention, in the cross section perpendicular to the longitudinal direction of the cells,

each exhaust gas emission cell and each exhaust gas introduction cell preferably have a polygonal cross section, and a side forming the cross sectional shape of each first exhaust gas introduction cell faces one of the exhaust gas emission cells, a side forming the cross sectional shape of each second exhaust gas introduction cell faces one of the exhaust gas emission cells, and the length of the side forming the cross sectional shape of the second exhaust gas introduction cell is not more than 0.8 times the length of the side forming the cross sectional shape of the first exhaust gas introduction cell.

The honeycomb filter having the above structure enables easier passage of exhaust gas through the cell walls separating the exhaust gas emission cells and the first exhaust gas introduction cells, effective suppression of the initial pressure loss, and prevention of an increase in the rate of increase of the pressure loss after accumulation of PMs.

If the ratio of the length of the side of the second exhaust gas introduction cell to the length of the side of the first exhaust gas introduction cell exceeds 0.8, the two sides do not have a big difference in length. Consequently, the initial pressure loss is hardly suppressed.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

preferably, the exhaust gas emission cells are each octagonal, the first exhaust gas introduction cells are each square, and the second exhaust gas introduction cells are each octagonal.

The honeycomb filter having the above structure has the same shape as the honeycomb filter in FIGS. 2A to 2C that is described concerning the effects thereof. Thus, the honeycomb filter can effectively suppress the initial pressure loss, have a large surface area for allowing PMs to accumulate thereon, and can maintain the pressure loss at a low level.

In the honeycomb filter according to the embodiments of the present invention, in the cross section perpendicular to the longitudinal direction of the cells,

preferably, the cross sectional area of each second exhaust gas introduction cell is equal in size to the cross sectional area of each exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is 40 to 70% the size of the cross sectional area of each second exhaust gas introduction cell.

The honeycomb structure having the above structure can provide difference between the resistance caused upon flowing of exhaust gas through the first exhaust gas introduction cells and the resistance caused upon flowing of exhaust gas through the second exhaust gas introduction cells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cells is less than 40% the size of the cross sectional area of the second exhaust gas introduction cells, the cross sectional area of the first exhaust gas introduction cells is too small. Consequently, high flow-through resistance occurs upon flowing of exhaust gas through the first exhaust gas introduction cells so that the pressure loss tends to be high. If the cross sectional area of the first exhaust gas introduction cells is more than 70% the size of the cross sectional area of the second exhaust gas introduction cells, the difference in the flow-through resistance of the first exhaust gas introduction cells and the flow-through resistance of the second exhaust gas introduction cells is small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the present invention, the cell walls defining rims of the cells preferably have a uniform thickness in any part of the honeycomb filter.

The honeycomb filter having this structure can exert the aforementioned effects through its entire body.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

preferably the exhaust gas emission cells have octagonal cross sections, the first exhaust gas introduction cells have square cross sections, and the second exhaust gas introduction cells have octagonal cross sections,

the cross sectional shape of each second exhaust gas introduction cell is congruent with the cross sectional shape of each exhaust gas emission cell, and

the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are arranged in the following manner:

the exhaust gas emission cells are each surrounded by alternately arranged four pieces of the first exhaust gas introduction cells and four pieces of the second exhaust gas introduction cells across the porous cell walls;

provided that a hypothetical segments connecting geometric centers of gravity of the octagonal cross sections of the four second exhaust gas introduction cells surrounding the exhaust gas emission cell are given, an intersection of the two segments crossing a shape region including the cross sectional shape of the exhaust gas emission cell is identical with a geometric center of gravity of the octagonal cross section of the exhaust gas emission cell; and

the four segments not crossing the shape region including the cross sectional shape of the exhaust gas emission cell form a square, and midpoints of the respective sides of the square are identical with geometric centers of gravity of the square cross sections of the four first exhaust gas introduction cells surrounding the exhaust gas emission cell,

a side facing the first exhaust gas introduction cell across a cell wall among the sides forming the cross sectional shape of the exhaust gas emission cell is parallel to a side facing the exhaust gas emission cell across the cell wall among the sides forming the cross sectional shape of the first exhaust gas introduction cell, and

a side facing the second exhaust gas introduction cell across a cell wall among the sides forming the cross sectional shape of the exhaust gas emission cell is parallel to a side facing the exhaust gas emission cell across the cell wall among the sides forming the cross sectional shape of the second exhaust gas introduction cell, and a side facing the second exhaust gas introduction cell across a cell wall among the sides forming the cross sectional shape of the first exhaust gas introduction cell is parallel to a side facing the first exhaust gas introduction cell across the cell wall among the sides forming the cross sectional shape of the second exhaust gas introduction cell, and distances between the parallel sides are the same.

In the honeycomb filter of the embodiments of the present invention, there are three patterns described below for the shape of the outer wall of the honeycomb fired body and the shape of the exhaust gas introduction cells adjacent to the outer wall;

(1) as shown in FIG. 10B and FIG. 12A, the thickness of the outer wall is not uniform; and the cross sectional shape of the first exhaust gas introduction cells adjacent to the outer call is the same as that of the first exhaust gas introduction cells not adjacent to the outer wall, and the cross sectional shape of the exhaust gas emission cells adjacent to the outer wall is the same as that of the exhaust gas emission cells not adjacent to the outer wall;

(2) as shown in FIG. 4A and FIG. 12B, the thickness of the outer wall is uniform; and the cross sectional shape of the first exhaust gas introduction cells adjacent to the outer wall is the same as that of the first exhaust gas introduction cells not adjacent to the outer wall, and the cross sectional shape of the exhaust gas emission cells adjacent to the outer wall is partially deformed in accordance with the line along the inner walls, which form the outer wall, of the first exhaust gas introduction cells adjacent to the outer wall, as compared to the cross sectional shape of the exhaust gas emission cells not adjacent to the outer wall; and

(3) as shown in FIG. 11B and FIG. 12C, the thickness of the outer wall is uniform in accordance with the cross sectional shapes of the first exhaust gas introduction cells and exhaust gas emission cells that are adjacent to the outer wall; and the cross sectional shape of the first exhaust gas introduction cells adjacent to the outer wall is the same as that of the first exhaust gas introduction cells not adjacent to the outer wall, and the cross sectional shape of the exhaust gas emission cells adjacent to the outer wall is the same as that of the exhaust gas emission cells not adjacent to the outer wall. In other words, the outer wall is curved in accordance with the cross sectional shapes of the first exhaust gas introduction cells and exhaust gas emission cells that are adjacent to the outer wall.

The honeycomb filter of the embodiments of the present invention is preferably such that the outer wall has corner portions, and a side, which contacts the outer wall, of each exhaust gas introduction cell and exhaust gas emission cell adjacent to the outer wall is straight and parallel to a side corresponding to an outer periphery of the outer wall in the cross section perpendicular to the longitudinal direction of the cells, in a manner that the thickness of the outer wall is uniform except for the corner portions.

A honeycomb filter having the above structure corresponds to the pattern (2) among the three patterns described above.

The outer wall enables to not only enhance the strength of the honeycomb fired body but also further reduce partial variations in the volume ratio in the exhaust gas emission cells and exhaust gas introduction cells in the honeycomb fired body, allowing the exhaust gas to flow more uniformly. As a result, the pressure loss can be reduced.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells all preferably have a square cross section.

Even in the case where the first exhaust gas introduction cells and the second exhaust gas introduction cells all have a square cross section, relations concerning the size, position, or the like of the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are different. For example, because the cross sectional area of each first exhaust gas introduction cell is smaller than the cross sectional area of each exhaust gas emission cell, the honeycomb filter of the embodiments of the present invention is different from the honeycomb filter 110 (see FIGS. 18A and 18B) in the aforementioned background art, and can exert the aforementioned effects of the embodiments of the present invention.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

preferably, the cross sectional area of each second exhaust gas introduction cell is equal in size to the cross sectional area of each exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is 40 to 70% the size of the cross sectional area of each second exhaust gas introduction cell.

The honeycomb filter having the above structure can provide difference between the resistance caused upon flowing of exhaust gas through the first exhaust gas introduction cells and the resistance caused upon flowing of exhaust gas through the second exhaust gas introduction cells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cells is less than 40% the size of the cross sectional area of the second exhaust gas introduction cells, the cross sectional area of the first exhaust gas introduction cells is too small. Consequently, high flow-through resistance occurs upon flowing of exhaust gas through the first exhaust gas introduction cells so that the pressure loss tends to be high. If the cross sectional area of the first exhaust gas introduction cells is more than 70% the size of the cross sectional area of the second exhaust gas introduction cells, the difference in flow-through resistance between the trust exhaust gas introduction cells and the second exhaust gas introduction cells is small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

preferably, the exhaust gas emission cells have a square cross section, the first exhaust gas introduction cells have a square cross section, and the second exhaust gas introduction cells have a square cross section,

the cross sectional shape of each second exhaust gas introduction cell is congruent with the cross sectional shape of each exhaust gas emission cell,

the exhaust gas emission cells, the first exhaust gas introduction-cells, and the second exhaust gas introduction cells are arranged in the following manner:

the exhaust gas emission cells are each surrounded by alternately arranged four pieces of the first exhaust gas introduction cells and four pieces of the second exhaust gas introduction cells across the porous cell walls;

provided that a hypothetical segments connecting geometric centers of gravity of the square cross sections of the four second exhaust gas introduction cells surrounding the exhaust gas emission cell are given, an intersection of the two segments crossing a shape region including the cross sectional shape of the exhaust gas emission cell is identical with a geometric center of gravity of the square cross section of the exhaust gas emission cell; and

the four segments not crossing the shape region including the cross sectional shape of the exhaust gas emission cell form a square, and midpoints of the sides of the square are identical with geometric centers of gravity of the square cross sections of the four first exhaust gas introduction cells surrounding the exhaust gas emission cell, and

a side facing the first exhaust gas introduction cell across a cell wall among the sides forming the cross sectional shape of the exhaust gas emission cell is parallel to a side facing the exhaust gas emission cell across the cell wall among the sides forming the cross sectional shape of the first exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cell wall among the sides forming the cross sectional shape of the first exhaust gas introduction cell is parallel to a side facing the first exhaust gas introduction cell across the cell wall among the sides forming the cross sectional shape of the second exhaust gas introduction cell, and distances between the parallel sides are the same.

In the honeycomb filter of the embodiments of the present invention, in the cross section perpendicular to the longitudinal direction of the aforementioned cells, preferably the vertex portions of the polygonal cells are formed by curved lines.

The honeycomb filter having cells with vertex portions formed by curved lines according to the above structure are not susceptible to concentration of stress derived from heat or the like in the corner portions of the cells. Thus, cracks hardly occur.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are preferably point-symmetrical polygons each having not more than eight sides.

The cells each having a point-symmetrical polygonal shape with not more than eight sides can reduce the resistance caused upon flowing of exhaust gas through the cells, and thus can further reduce the pressure loss.

In the honeycomb filter according to the embodiments of the present invention, preferably, the exhaust gas emission cells and the exhaust gas introduction cells are each in a shape formed by curved lines in the direction perpendicular to the longitudinal direction of the cells, and the thickness of the cell walls separating the first exhaust gas introduction cells and the respective exhaust gas emission cells is smaller than the thickness of the cell walls separating the second exhaust gas introduction cells and the respective exhaust gas emission cells.

In the honeycomb filter according to the embodiments of the present invention, in a case where the thickness of the cell walls separating the first exhaust gas introduction cells and the respective exhaust gas emission cells is smaller than the thickness of the cell walls separating the second exhaust gas introduction cells and the respective exhaust gas emission cells, exhaust gas easily passes through the cell walls separating the first exhaust gas introduction cells and the respective exhaust gas emission cells at an early stage. After accumulation of a certain amount of PMs, exhaust gas is likely to pass through the cell walls separating the second exhaust gas introduction calls and the respective exhaust gas emission cells. The second exhaust gas introduction cells have a larger cross sectional area than the first exhaust gas introduction cells, and the exhaust gas emission cells have an equal or larger cross sectional area than the respective second exhaust gas introduction cells. For this reason, the above effects of the embodiments of the present invention are exerted.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

preferably the exhaust gas introduction cells and the exhaust gas emission cells are each in a shape formed by curved lines, and the thickness of the cell walls separating the first exhaust gas introduction cells and the respective exhaust gas emission cells is 40 to 75% the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells.

In the honeycomb filter of the embodiments of the present invention, if the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is 40 to 75% the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, exhaust gas easily passes through the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells at an initial stage. Then, after accumulation of a certain amount of PMs, exhaust gas passes through the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells. Moreover, the second exhaust gas introduction cells each have a larger cross sectional area than each first exhaust gas introduction cell, and each exhaust gas emission cell has an equal or larger cross sectional area than each second exhaust gas introduction cell. Thus, the aforementioned effects of the embodiments of the present invention can be exerted.

If the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is less than 40% the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells need to be extremely thin. Consequently, the honeycomb filter has low mechanical strength. If the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is more than 75% the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, the former cells and latter cells do not have a big difference in the thickness. Consequently, the aforementioned pressure loss reduction effect may not be obtained.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are preferably round.

In the honeycomb filter having the above structure, if the cross sectional shapes of the exhaust gas emission cells, first exhaust gas introduction cells, and the second exhaust gas introduction cells are all round, the effects of the embodiments of the present invention can be exerted.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

the exhaust gas emission cells and the second exhaust gas introduction cells each preferably have a convex square cross section formed by four outwardly curved lines, whereas the first exhaust gas introduction cells each preferably have a concave square cross section formed by four inwardly curved lines.

The term “convex square” herein refers to a shape formed by four outwardly curved lines in the same length. The shape seems to be a square having sides expanding outwardly from the geometrical center of gravity. The term “concave square” herein refers to a shape formed by four inwardly curved lines in the same length. The shape seems to be a square having sides bending toward the geometrical center of gravity.

The exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells in this honeycomb filter having this structure have the aforementioned structures. Thus, the exhaust gas emission cells each have a larger cross sectional area than each first exhaust gas introduction cells, and thereby providing a honeycomb filter having the relations concerning the sizes of the exhaust gas emission cells, first exhaust gas introduction cells, and second exhaust gas introduction cells according to the embodiments of the present invention. Consequently, the effects of the embodiments of the present invention can be exerted.

In the honeycomb filter according to the embodiments of the present invention, in the direction perpendicular to the longitudinal direction of the cells,

the cross sectional area of each second exhaust gas introduction cell is preferably equal in size to the cross sectional area of each exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is preferably 40 to 70% the size of the cross sectional area of each second exhaust gas introduction cell.

The honeycomb filter having the above structure can provide difference between the resistance caused upon flowing of exhaust gas through the first exhaust gas introduction cells and the resistance caused upon flowing of exhaust gas through the second exhaust gas introduction cells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cells is less than 40% the size of the cross sectional area of the second exhaust gas introduction cells, the cross sectional area of the first exhaust gas introduction cells is too small. Consequently, high flow-through resistance occurs upon flowing of exhaust gas through the first exhaust gas Introduction cells so that the pressure loss tends to be high. If the cross sectioned area of the first exhaust gas introduction cells is more than 70% the size of the cross sectional area of the second exhaust gas introduction cells, the difference between the flow-through resistance of the first exhaust gas introduction cells and the flow-through resistance of the second exhaust gas introduction cells is small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter of the embodiments of the present invention, the exhaust gas introduction cells preferably consist only of the first exhaust gas introduction cells and the second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells.

This is because a smaller muster of first exhaust gas introduction cells each having a smaller cross sectional area than each second exhaust gas introduction cell provides a larger effective introduction cell area, thereby allowing PMs to thinly and widely accumulate.

The honeycomb filter of the embodiments of the present invention preferably includes a plurality of honeycomb fired bodies combined with one another by adhesive layers residing therebetween, the honeycomb fired bodies each having the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells, and each having an outer wall on the periphery thereof.

With regard to the structure formed by combining a plurality of honeycomb fired bodies to one another with adhesive layers residing therebetween, because the cells forming one honeycomb fired body have the structures according to the embodiments of the present invention, an aggregate of the honeycomb fired bodies can exert the effects of the embodiments of the present invention.

Moreover, the honeycomb fired bodies each having a smaller volume enables to reduce the thermal stress that is generated upon production and usage thereof, preventing damage such as cracks.

According to the honeycomb filter of the embodiments of the present invention, the aforementioned honeycomb filter includes honeycomb fired bodies, and the honeycomb fired body preferably includes silicon carbide or silicon-containing silicon carbide.

The silicon carbide and silicon-containing silicon carbide are materials excellent in heat resistance. Thus, the honeycomb filter is better in heat resistance.

In the honeycomb filter of the embodiments of the present invention, the thickness of the cell wall of the honeycomb filter is preferably 0.10 to 0.46 mm.

The cell wall having such a thickness is enough thick to capture PMs in the gas and effectively suppresses an increase of the pressure loss. As a result, the honeycomb filter of the embodiments of the present invention can sufficiently exert the effects of the embodiments of the present invention.

If the thickness is less than 0.10 mm, the cell wall is too thin, so that the mechanical strength of the honeycomb filter is lowered. If the thickness is more than 0.46 mm, the cell wall is too thick, so that the pressure loss upon passage of exhaust gas through the cell wall becomes greater.

In the honeycomb filter of the embodiments of the present invention, the porosity of the cell walls is preferably 40 to 65%.

If the cell walls have a porosity of 40 to 65%, the increase in the pressure loss due to carrying of SCR catalyst can be suppressed. In addition, the cell walls can favorably capture PMs in exhaust gas, and the increase in the pressure loss due to the cell walls can be suppressed. Thus, in the provided honeycomb filter, the pressure loss is small at the initial stage and does not easily increase even after accumulation of PMs.

In the case of the cell walls having a porosity of less than 40%, the pressure loss tends to be large once the SCR catalyst is carried because the proportion of the pores in the cell walls is too small. On the other hand, in the case of the cell walls having a porosity of more than 65%, the cell walls have poor mechanical characteristics, and thus cracks tend to occur during regeneration or the like.

In the honeycomb filter of the embodiments of the present invention, the average pore diameter of the pores in the cell walls is preferably 8 to 25 μm. The pore diameter and the porosity are measured by a mercury injection method under conditions of the contact angle of 130 degrees and the surface tension of 485 mN/m.

If the average pore diameter of the pores in the cell walls is less than 8 μm, the pressure loss will increase once the SCR catalyst is carried. On the other hand, if the average pore diameter of the pores in the cell walls is more than 25 μm, PM capturing efficiency will decrease because the porosity is too large.

The honeycomb filter of the embodiments of the present invention preferably has a periphery coat layer formed on the periphery thereof.

The periphery coat layer serves to mechanically protect the cells located inside. As a result, the honeycomb filter has excellent mechanical characteristics such as compression strength.

In regard to the cross sectional shape perpendicular to the longitudinal direction of the cells forming the honeycomb filter, the honeycomb filter of the embodiments of the present invention is preferably such that the first exhaust gas introduction cells, the second exhaust gas introduction cells, and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side; the cross sectional shape of the first exhaust gas introduction cells is different from the cross sectional shape of the second exhaust gas introduction cells; and the cross sectional shape of the exhaust gas emission cells is different from the cross sectional shape of the first exhaust gas introduction cells. The cross sectional shapes being different. Here, “different” means “not congruent,” but encompasses “similar”. In other words, if the cross sectional shapes are similar to each other, the cross sectional shapes are considered different from each other.

Each first exhaust gas introduction cell itself has a uniform cross sectional shape at any cross section thereof, each second exhaust gas introduction cell itself has a uniform cross sectional shape at any cross section thereof, and each exhaust gas emission cell has a uniform cross sectional shape at any cross section thereof. Each first exhaust gas introduction cell has a different cross sectional shape from each second exhaust gas introduction cell. Each exhaust gas emission cell has a different cross sectional shape from each first exhaust gas introduction cell.

The honeycomb filter of the embodiments of the present invention preferably has the following structure. In the cross section perpendicular to the longitudinal direction of the cells, a cell unit having a cell structure described below is two-dimensionally repeated, where the first exhaust gas introduction cells and the second exhaust gas introduction cells surrounding each exhaust gas emission cell in the unit are shared between adjacent cell units.

cell structure: each exhaust gas emission cell is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells, and each exhaust gas emission cell has an equal or larger cross sectional area than each second exhaust gas introduction cell in a direction perpendicular to the longitudinal, direction of the cells, and

the exhaust gas introduction cells and the exhaust gas emission cells have the following feature A or B in the cross section perpendicular to the longitudinal direction of the cells:

A: the exhaust gas introduction cells and the exhaust gas emission cells are each polygonal, and

a side forming the cross sectional shape of each first exhaust gas introduction cell faces one of the exhaust gas emission cells, a side forming the cross sectional shape of each second exhaust gas introduction cell faces one of the exhaust gas emission cells, and the side of the first exhaust gas introduction cell is longer than the side of the second exhaust gas introduction cell, or

a side forming the cross sectional shape of each first exhaust gas introduction cell faces one of the exhaust gas emission cells, and none of the sides forming the cross sectional shape of each second exhaust gas introduction cell faces the exhaust gas emission cells;

B: the exhaust gas introduction cells and the exhaust gas emission cells are each in a shape formed by a curved line, and

the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is smaller than the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells.

Such a honeycomb filter is preferable because the two-dimensional repetition of the cell unit forms a filter having a large capacity. The filter has an outer wall, and the cell units naturally do not spread outside the outer wall. The cell units are cut out properly to fit into the shape of the outer wall.

The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.

The present invention is not limited to those embodiments, surd may be modified within a scope not changing the gist of the present invention.

First Embodiment

The following will discuss a honeycomb filter of the first embodiment of the present invention.

The honeycomb filter according to the first embodiment of the present invention includes a plurality of honeycomb fired bodies. Each honeycomb fired body includes porous cell walls that define rims of a plurality of cells that serve as the channels of exhaust gas; exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side; and exhaust gets introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells. Each honeycomb fired body has an outer wall on the periphery thereof, and a plurality of honeycomb fired bodies are combined with one another by adhesive layers residing therebetween.

In regard to the cells other than the cells adjacent to the outer wall, the exhaust gas emission cells are each adjacently surrounded fully by the first exhaust gas introduction cells and the second exhaust gas introduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of the cells, each second exhaust gas introduction cell has a larger cross sectional area than each first exhaust gas introduction cell, and each exhaust gas emission cell has the same cross sectional area as each second exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells and the exhaust gas introduction cells are each polygonal, and a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the first exhaust gas introduction cell is longer than a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the second exhaust gas introduction cell.

The cells adjacent to the outer wall include the first exhaust gas introduction cells and the exhaust gas emission cell arranged alternately with the first exhaust gas introduction cells. The cross sectional area of each exhaust gas emission cell in a direction perpendicular to the longitudinal direction of the cell is larger than the cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells.

Specifically, the arrangement of two first exhaust gas introduction cells, one second exhaust gas introduction cell, and one exhaust gas emission cell is two-dimensionally repeated; the outer wall has corner portions, and a side, which contacts the outer wall, of each first exhaust gas introduction cell and exhaust gas emission cell adjacent to the outer wall is straight and parallel to a side corresponding to an outer periphery of the outer wall in the cross section perpendicular to the longitudinal direction of the cells, in a manner that the thickness of the outer wall is uniform except for the corner portions; and the exhaust gas emission cells adjacent to the outer wall are partially deformed. The cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller.

The exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion, in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side.

FIG. 3 is a perspective view schematically illustrating one example of the honeycomb filter according to the first embodiment of the present invention.

FIG. 4A is a perspective view schematically illustrating one example of the honeycomb fired body forming the honeycomb filter shown in FIG. 3. FIG. 4B is an A-A line cross sectional vies of the honeycomb fired body shown in FIG. 4A.

The honeycomb filter 20 shown in FIG. 3 includes a ceramic block 18 formed by combining a plurality of honeycomb fired bodies 10 with an adhesive layer 15 residing therebetween, and has a periphery coat layer 16 for preventing leakage of emission gas on the periphery of the ceramic block 18. The periphery coat layer 16 may be optionally formed.

Such a honeycomb filter including combined honeycomb fired bodies is also referred to as an aggregated honeycomb filter.

The honeycomb fired body 10 has a rectangular pillar shape that is chamfered so that the corner portions at end faces thereof are formed by curved lines as shown in FIG. 4A. This prevents thermal stress concentration at the corner portions to thereby prevent occurrence of damages such as cracks. The corner portions each may be chamfered in a manner to have a shape formed by straight lines.

In the honeycomb filter 20 according to the first embodiment, the exhaust gas emission cells each have an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side, and the exhaust gas introduction cells each have an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side. The plug is preferably made of the same material as the material of the honeycomb fired body.

In the honeycomb fired body 10 shown in FIG. 4A and FIG. 4B, the exhaust gas emission cells 11 having an octagonal cross section are each adjacently surrounded fully by the first exhaust gas introduction cells 12 each having a square cross section and the second exhaust gas introduction cells 14 each having an octagonal cross section across porous cell walls therebetween. The first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 are alternately arranged around each exhaust gas emission cell 11. Each second exhaust gas introduction cell 14 has a larger cross sectional area than each first exhaust gas introduction cell 12, and each exhaust gas emission cell 11 has the same cross sectional area as each second exhaust gas introduction cell 14. An outer wall 17 is formed on the periphery of the honeycomb fired body 10, and the cells adjacent to the outer wall 17 include exhaust gas emission cells 11A and 11B and first exhaust gas introduction cells 12A.

Each second exhaust gas introduction cell 14 and each exhaust gas emission cell 11 have octagonal cross sections, and the cross sections are congruent with each other.

In the honeycomb filter 20 according to the present embodiment, a side, which contacts the outer wall 17, of each of the exhaust gas introduction cells 11A and 11B and the exhaust gas emission cells 12A adjacent to the outer wall 17 is straight and parallel to a side corresponding to the outer periphery of the outer wall 17 in the cross section perpendicular to the longitudinal direction of the cells, in a manner that the thickness of the outer wall 17 is uniform except for the corner portions of the honeycomb fired body.

The cross sections of the exhaust gas emission cells 11A adjacent to the outer wall 17 are partially cut so that the cross sectional shapes are changed from octagons to hexagons. The cross sectional shape of the first exhaust gas introduction cells 12A may be in a partially cut shape but is preferably congruent with the cross sectional shape of the first exhaust gas introduction cells 12.

The cross sections of exhaust gas introduction cells 11B at the corner portions of the honeycomb fired body 10 have been changed from octagon to substantially pentagon having chamfered portions 110B formed by curved lines. The chamfered portions 110B of the exhaust gas emission cells 11B illustrated in FIG. 4A are chamfered to be curved but may be linearly chamfered.

The aforementioned structure enables to not only enhance the strength of the honeycomb fired body by the outer wall but also further reduce partial variation in the volume ratio between the exhaust gas emission cells and the exhaust gas introduction cells in the honeycomb fired body, allowing the exhaust gas to flow more uniformly. In the vicinity of the outer wall, exhaust gas smoothly flows into the first exhaust gas introduction cells 12 and the cell wall and outer wall effectively function as a filter, so that the pressure loss can be reduced.

In the honeycomb fired body 10, the exhaust gas emission cells 11A and 11B and the exhaust gas introduction cells 12A are alternately arranged along the outer wall 17, and the exhaust gas emission cells 11 is orderly arranged inside the outer wall with the first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 residing therebetween. As described above, the exhaust gas emission cells 11, the first exhaust gas introduction cells 12, and the second exhaust gas introduction cells 14 are arranged in a very ordered manner.

Each exhaust gas emission cell 11 and each second exhaust gas introduction cell 14 have the same octagonal shape. The octagon is point-symmetry with respect to the center of gravity. In the octagon, all hypotenuse sides (14 a in FIGS. 2A to 2C) have the same length, and all vertical or horizontal sides (14 b in FIGS. 2A to 2C) have the same length. Moreover, four first sides (the hypotenuse sides) and four second sides (the vertical or horizontal sides) are alternately arranged. The first sides and the second sides form an angle of 135°.

The term “hypotenuse side” generally refers to the longest side that is opposite to the right angle in a right-angled triangle. However, for convenience for explanation, a “hypotenuse side” herein refers to a side such as the side 14 a and the side 11 b that has a certain angle, except for 90° and 0°, to below-mentioned four hypothetical segments. For differentiation from this term, the term “vertical or horizontal side” herein refers to a side such as the side 14 b and the side 11 a that is parallel to or vertical to the below-mentioned, four hypothetical segments.

The hypothetical segments mentioned in the explanation of the terms “hypotenuse side” and “vertical or horizontal sides” refer to four hypothetical segments (the four segments form a square) that do not cross the cross sectional figure of the exhaust gas emission cell 11, among hypothetical segments that connect geometric centers of gravity of the respective cross sectional figures of the four second exhaust gas introduction cells arranged around the exhaust gas emission cell 11.

Each first exhaust gas introduction cell 12 has a square cross section.

In the cross section of the three kinds of cells which are adjacent to each other, namely, the exhaust gas emission cell 11, the second exhaust gas introduction cell 14, and the first exhaust gas introduction cell 12, the side 11 a facing the first exhaust gas introduction cell 12 across the cell wall 13 among the sides of the octagonal exhaust gas emission cell 11 is parallel to the side 12 a facing the exhaust gas emission cell 11 across the cell wall 13 among the sides of the square first exhaust gas introduction cell 12.

Also, the side 11 b facing the octagonal second exhaust gas introduction cell 14 across the cell wall 13 among the sides of the octagonal exhaust gas emission cells 11 is parallel to the side 14 a facing the exhaust gas emission cell 11 across the cell wall 13 among the sides of the octagonal second exhaust gas introduction cell 14. Moreover, the side 12 b facing the second exhaust gas introduction cell 14 across the cell wall 13 among the sides of the first exhaust gas introduction cell 12 is parallel to the side 14 b facing the first exhaust gas introduction cell 12 across the cell wall 13 among the sides of the second exhaust gas introduction cell 14. Furthermore, the distances between the parallel sides of all the above pairs are the same (see FIGS. 2A to 2C). That is, the distance between the parallel sides 11 a and 12 a, the distance between the parallel sides 11 b and 14 a, and the distance between the parallel sides 12 b and 14 b are line same.

Additionally, the exhaust gas emission cells 11, the first exhaust gas introduction cells 12, and the second exhaust gas introduction cells 14 are arranged in a manner satisfying the conditions below.

Among the hypothetical segments connecting the geometric centers of gravity of the octagonal shapes of the four second exhaust gas introduction cells 14 surrounding the exhaust gas emission cell 11, an intersection of the two segments crossing the octagonal shape region of the exhaust gas emission cell 11 is identical with the geometric center of gravity of the octagonal cross section of the exhaust gas emission cell 11.

Moreover, among the hypothetical segments connecting the geometric centers of gravity of the octagonal shapes of the four second exhaust gas introduction cells 14, the four segments not crossing the octagonal shape region of the exhaust gas emission cell 11 forms a square, and midpoints of the respective sides of the square are identical with the geometric centers of gravity of the respective square shapes of the four first exhaust gas introduction cells 12 surrounding the exhaust gas emission cell 11.

As described above, the octagonal exhaust gas emission cell 11 is adjacently surrounded by alternately arranged four pieces of the first square exhaust gas introduction cells 12 and four pieces of the second octagonal exhaust gas introduction cells 14 across the cell walls 13 to form a single unit. The unit is two-dimensionally repeated, where the first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 in the unit are shared between adjacent cell units, to form a honeycomb filter. Since the units share the first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14, the first exhaust gas introduction cell 12 and the second exhaust gas introduction cell 14, that adjoin the exhaust gas emission cell 11 across the cell walls 13, also adjoining the exhaust gas emission cell 11 in the adjacent unit across the cell wall 13.

FIG. 5 is an enlarged cross sectional view perpendicular to the longitudinal direction of the honeycomb filter. FIG. 5 illustrates now each cell unit (cell structure) is two-dimensionally, i.e. in X and Y directions shown in FIG. 5, repeated in the case where the second exhaust gas introduction cells 14 and the exhaust gas emission cells 11 are octagonal, the first exhaust gas introduction cells 12 are square in the cross section of the cells, and the aforementioned conditions are satisfied, and also illustrates how the first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 are shared between the cell units (cell structure).

A cell unit 1, a cell unit 2, and a cell unit 3 each have a structure in which the exhaust gas emission cell 11 is fully surrounded by alternately arranged four pieces of the first exhaust gas introduction cells 12 and four pieces of the second exhaust gas introduction cells 14 across the cell walls 13 in a manner satisfying the aforementioned conditions. The cell unit 2 has the same structure as that of the cell unit 1. The cell unit 2 is adjacent to the cell unit 1 in the X direction while snaring one piece of the first exhaust gas introduction cell 12 and two pieces of the second exhaust gas introduction cells 14 with the cell unit 1. The cells shared between the cell unit 1 and the cell unit 2 are depicted as “shared portion 2” in FIG. 5. The cell unit 3 has the same structure as that of the cell unit 1. The cell unit 3 is adjacent to the cell unit 1 in the Y direction while sharing one piece of the first exhaust gas introduction cell 12 and two pieces of the second exhaust gas introduction cells 14 with the cell unit 1. The cells shared between the cell unit 1 and the cell unit 3 are depicted as “shared portion 1” in FIG. 5.

Meanwhile, FIG. 5 shows four segments H, I, J, and K that do not cross the octagonal shape region of the exhaust gas emission cell 11, and hypothetical two segments L and M that cross the octagonal shape region of the exhaust gas emission cell 11, among hypothetical segments connecting the geometric centers of gravity of the respective octagonal shapes of the four pieces of the second exhaust gas introduction cells 14. The “shared portion 2” is depicted by cross-hatching with segments in the same direction as that of the segment M, and the “shared portion 1” is depicted by cross-hatching with segments in the same direction as that of the segment L.

As shown in FIG. 5, an intersection of the two segments L and M is identical with the geometric center of gravity of the exhaust gas emission cell 11.

With regard to the cell shapes in the honeycomb filter 20 shown in FIGS. 2A, to 2C, 3, 4A, and 4B, excluding the exhaust gas emission cells 11A and 11B adjacent to the outer wall 17, the exhaust gas emission cells 11 and the second exhaust gas introduction cells 14 each have an octagonal cross section, and the first exhaust gas introduction cells 12 and 12A each hate a square cross section. However, the cross sectional shapes of the exhaust gas emission cells and the exhaust gas introduction cells of the present invention are not limited to the above shapes, and may be all square as mentioned belong or may be combinations of other polygons.

Moreover, the exhaust gas emission cells 11, the first exhaust gas introduction cells 12, and the second exhaust gas introduction cells 14, which are polygons in the cross section thereof, may be roundly chamfered so that the vertex portions are formed by curved lines in the cross section.

Examples of the curved lines include curved lines obtained by dividing a circle into quarters, and curved lines obtained by dividing an ellipse into four equal parts linearly along the long axis and the axis perpendicular to the long axis. In particular, the vertex portions of the cells having a rectangular cross section are preferably chamfered to be formed by curved lines in the cross section. This prevents stress concentration at the vertex portions, thereby preventing cracks in the cell walls.

Furthermore, the honeycomb filter 20 may partially include cells formed by curved lines such as a cell having a circle cross section.

The exhaust gas emission cells 11A and 11B adjacent to the outer wall 17 are excluded in the description below.

The cross sectional area of each first exhaust gas introduction cell 12 is 0.7 mm² or smaller, and the proportion of the cross sectional area of each first exhaust gas introduction cell 12 relative to the cross sectional area of each second exhaust gas introduction call 14 is preferably 40 to 70% and more preferably 50 to 65%.

In the honeycomb filter 20 shown in FIGS. 2A to 2C, 3, 4A, and 4B, the cross sectional area of each exhaust gas emission cell 11 is equal to the cross sectional area of each second exhaust gas introduction cell 14; however, the cross sectional area of each exhaust gas emission cell 11 may be larger than the cross sectional area of each second exhaust gas introduction cell 14.

The cross sectional area of each exhaust gas emission cell 11 is probably 1.05 to 1.5 times the size of the cross sectional area of each second exhaust gas introduction cell 14.

Moreover, the side 12 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the first exhaust gas introduction cell 12 is longer than the side 14 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14. The sides 12 a and 14 a are sides facing the exhaust gas emission cell 11 according to the aforementioned definition of the embodiments of the present invention.

The ratio of the length of the side 14 a of the second exhaust gas introduction cell 14 to the length of the side 12 a of the first exhaust gas introduction cell 12 (length of the side 14 a/length of the side 12 a) is not particularly limited, and is preferably not more than 0.8, more preferably not more than 0.7, and still more preferably not more than 0.5.

As shown in FIG. 4B, exhaust gas G₁ (exhaust gas is represented by an arrow G₁ which shows the flow of exhaust gas in FIG. 4B) having flowed into the first exhaust gas introduction cells 12 or the second exhaust gas introduction cells 14 inevitably passes through the cell walls 13 which separates the exhaust gas emission cells 11 and the respective first exhaust gas introduction cells 12 or the respective second exhaust gas introduction cells 14, and then flows out of the exhaust gas emission cells 11. Upon passage of exhaust gas G₁ through the cell walls 13, PMs and the like in the exhaust gas are captured so that the cell walls 13 function as filters.

The exhaust gas emission cells 11, the first exhaust gas introduction cells 12, and the second exhaust gas introduction cells 14 allow flow of gas such as exhaust gas as described above. For the flow of exhaust gas in the direction shown in FIG. 4B, an end at a first end face 10 f of the honeycomb fired body 10 (the end on the side at which the exhaust gas emission cells 11 are plugged) is referred to as an exhaust gas introduction side end, an end at a second end face 10 r of the honeycomb fired body 10 (the end on the side at which the first exhaust gas introduction cells 12, and the second exhaust gas introduction cells 14 are plugged) is referred to as an exhaust gas emission side end.

As described in the section for the effects of the honeycomb filter according to the embodiments of the present invention, the honeycomb filter 20 having the above structure has the following effects: the cross sectional area of each exhaust gas emission cell is large so that the passage resistance upon passage of exhaust gas through exhaust gas emission cells and the outflow resistance upon emission of exhaust gas from the honeycomb filter are small; and the cross sectional area of each second exhaust gas introduction cell is larger than the cross sectional area of each first exhaust gas introduction cell so that PMs accumulate widely and thinly in the first exhaust gas introduction cells and the second exhaust gas introduction cells, and the pressure loss can be maintained small until the amount of accumulated PMs reaches 6 to 10 g/L. At the same time, the cross section area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller so that the openings of the first exhaust gas introduction cells become small as the amount of accumulated PM reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. Consequently, the regeneration treatment can be reliably carried out at an appropriate time. Thus, in the honeycomb filter according to the present embodiment, the excessive capture and the increase in the amount of heat generated by regeneration are suppressed, and the honeycomb filter is thus less likely to have cracks and dissolution loss.

FIG. 6 is an end face view illustrating an end face of the honeycomb filter 20 shown in FIG. 3 and FIGS. 4A and 4B.

In the honeycomb filter 20 having the structure shown in FIG. 6, the thickness of the outer wall of the honeycomb fired body 10 forming the honeycomb filter is uniform throughout the outer wall 17. Thus, the outer wall 17 enables to not only enhance the strength of the honeycomb fired body and but also further reduce partial variations in the volume ratio in the exhaust gas emission cells and exhaust gas introduction cells in the honeycomb fired body, allowing the exhaust gas to flow more uniformly. As at result, the pressure loss can be reduced.

In addition, in regard to the cells adjacent to the outer wall 17, because the exhaust gas emission cells 11A and 11B and are each adjacent to the first exhaust gas introduction cell 12A, exhaust gas can pass through the inside of the outer wall 17, as shown in FIG. 6, which means that part of the outer wall 17 can be used as a filter. As a result, the pressure loss can be further reduced.

Moreover, because the outer wall 17 is chamfered, stress is prevented from concentrating on the corner portions of the honeycomb fired body 10, so that cracks hardly occur in the corner portions of the honeycomb fired body 10. When the cross sectional shape of the exhaust gas emission cell 11B positioned at the corner portion is a pentagon formed by straight lines, not having the chamfered portion 110B formed by curved lines, the exhaust gas emission cell 11B positioned close to the corner portion of the honeycomb fired body 10 is likely to have a stress concentrated thereon to easily have cracks. In the honeycomb filter 20, however, because the exhaust gas emission cells 11B each have the chamfered portion 110B, cracks hardly occurs therein.

The honeycomb filter 20 according to the first embodiment includes a plurality of honeycomb fired bodies 10. Examples of the material for the honeycomb fired bodies 10 include carbide ceramics such as silicon carbide, titanium carbide, tantalum carbide, and tungsten carbide; nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; oxide ceramics such as alumina, zirconia, cordierite, mullite, and aluminum titanate; and silicon-containing silicon carbide. Silicon carbide or silicon-containing silicon carbide is preferable among the examples as they are excellent in heat resistance, mechanical strength, thermal conductivity and the like.

Meanwhile, the silicon-containing silicon carbide is a mixture of a silicon carbide and silicon metal, and is preferably silicon-containing silicon carbide including 60 wt % or more of silicon carbide.

The thickness of the cell walls separating the cells is preferably uniform at any part in the honeycomb fired body 10 forming the honeycomb filter 20 according to the first embodiment. The thickness of the cell walls is preferably 0.10 to 0.46 mm, and more preferably 0.12 to 0.35 mm. The thickness of the outer wall 17 is preferably 0.10 to 0.50 mm Meanwhile, the thickness of the cell wall is a value measured as the thickness D shown in FIG. 1 based on the aforementioned definition.

The thickness of the plug is preferably 1.0 to 5.0 mm.

In the honeycomb filter of the embodiments of the present invention, the porosity of the cell walls is preferably 40 to 65%.

If the cell walls have a porosity of 40 to 65%, the cell walls can favorably capture PMs in exhaust gas, and the increase in the pressure loss due to the cell walls can be suppressed. Thus, in the provided honeycomb filter, the pressure loss is small at the initial stage and does not easily increase even after accumulation of PMs.

In the case of the cell walls having a porosity of less than 40%, the pressure loss tends to be large at the time of passage of exhaust gas through the cell walls because the proportion of the pores in the cell walls is too small and exhaust gas thus does not easily pass through the cell walls. On the other hand, in the case of the cell walls having a porosity of more than 65%, the cell walls have poor mechanical characteristics, and thus cracks tend to occur during regeneration or the like.

The average pore diameter of pores in the cell walls is preferably 8 to 25 μm in the honeycomb fired body 10 forming the honeycomb filter 20 according to the first embodiment. The pore diameter and the porosity are measured by a mercury injection method under conditions of the contact angle of 130 degrees the surface tension of 435 mN/m.

The honeycomb filter having the above structure can capture PMs at a high capturing efficiency while suppressing an increase in the pressure loss. If the average pore diameter of the pores in the cell walls is less than 8 μm, the pressure loss upon passage of exhaust gas through the cell walls will increase because the pores are too small. If the average pore diameter of the pores in the cell walls is more than 25 μm, the PM capturing efficiency will decrease because the porosity is too large.

The number of the cells per unit area is preferably 31 to 62 pcs/cm² (200 to 400 pcs/inch²) in the cross section of the honeycomb fired body 10.

The honeycomb filter 20 according to the embodiment of the present invention is formed by combining, with an adhesive layer residing therebetween, a plurality of the honeycomb fired bodies each having an outer wall on the periphery thereof. An adhesive layer that combines the honeycomb fired bodies is prepared by applying an adhesive paste that contains an inorganic binder and inorganic particles, and drying the adhesive paste. The adhesive layer may further contain at least one of an inorganic fiber and a whisker.

The adhesive layer preferably has a thickness of 0.5 to 2.0 mm.

The honeycomb filter according to the first embodiment of the present invention may have a periphery coat layer on the periphery thereof. The material of the periphery coat layer is preferably the same as the materials of the adhesive layer.

The periphery coat layer preferably has a thickness of 0.1 to 3.0 mm.

The following will describe a method of manufacturing the honeycomb filter according to the first embodiment of the present invention.

The method described below uses a silicon carbide as ceramic powder.

(1) A molding process for manufacturing a honeycomb molded body is performed by extrusion molding a wet mixture containing ceramic powder and a binder.

Specifically, silicon carbide powders having different average particle sizes serving as ceramic powder, an organic binder, a pore-forming agent, a liquid plasticizer, a lubricant, and water are mixed to prepare a wet mixture for manufacturing a honeycomb molded body.

Examples of a pore-forming agents include balloons that are fine hollow spheres including oxide-based ceramics, spherical acrylic particles, graphite, and starch.

The balloons are not particularly limited, and examples thereof include alumina balloon, glass micro balloon, shirasu balloon, fly ash balloon (FA balloon), and mullite balloon. Alumina balloon is preferable among these.

The average particle diameter or the pore-forming agent is preferably 30 to 50 μm, and more preferably 35 to 45 μm. It is still more preferred that the average particle diameter of the pore-forming agent is 1.5 to 2.5 times the intended average pore diameter of the honeycomb filter. In addition, the pore-forming agent having a pore diameter of 20 μm or less accounts for preferably 10% or less, and more preferably 5% or less of all the particles of the pore-forming agent; and the pore-forming agent having a pore diameter of 60 μm or more accounts for preferably 10% or less, and more preferably 5% or less of all the particles of the pore-forming agent. Further, the pore-forming agent is mixed preferably in a proportion of 10 to 25 wt %, and more preferably 15 to 20 wt % relative to silicon carbide powder used as ceramic powder.

Then, the wet mixture is charged into an extrusion molding machine and extrusion-molded to manufacture honeycomb molded bodies in predetermined shapes.

Here, a honeycomb molded body is manufactured with a die that can make a cross sectional shape having the cell structure (shapes and arrangement of the cells) shown in FIG. 4A. Herein, the size of the first exhaust gas introduction cells 12 is set such that the cross sectional area of each first exhaust gas introduction cell 12 is 0.7 mm² or smaller.

(2) The honeycomb molded bodies are cut at a predetermined length and dried with use of a drying apparatus such as a microwave drying apparatus, a hot-air drying apparatus, a dielectric drying apparatus, a reduced-pressure drying apparatus, a vacuum drying apparatus, or a freeze drying apparatus. Then, predetermined cells are plugged by filling the cells with a plug material paste to be a plug material (plugging process).

Here, the wet mixture may be used as the plug material paste.

(3) Then, the honeycomb molded body is heated at 300° C. to 650° C. in a depressing furnace to remove organic matters in the honeycomb molded body (decreasing process). The degreased honeycomb molded body is transferred to a firing furnace and fired at 2000° C. to 2200° C. (firing process). In this manner, the honeycomb fired body having the shape as shown in FIG. 4A is manufactured.

The plug material paste filled into the end of the cells is fired by heat to be a plug material.

Conditions for cutting, drying, plugging, degreasing, and firing may be conditions conventionally used for manufacturing honeycomb fired bodies.

(4) A plurality of the honeycomb fired bodies are stacked in series with the adhesion paste residing therebetween on a support table to combine the honeycomb fired bodies (combining process) so that a honeycomb aggregate body including the plurality of stacked honeycomb fired bodies is manufactured.

The adhesive paste contains, for example, an inorganic binder, an organic binder, and inorganic particles. The adhesive paste may further contain inorganic fibers and/or whisker.

Examples of the inorganic particles in the adhesive paste include carbide particles, nitride particles, and the like. Specific examples thereof include inorganic particles made from silicon carbide, silicon nitride, boron nitride, and the like. Each of these may be used alone, or two or more of these may be used in combination. Among the inorganic particles, silicon carbide particles are preferable due to their superior thermal conductivity.

Examples of the inorganic fibers and/or whisker in the adhesive paste include inorganic fibers and/or whisker made from silica-alumina, mullite, alumina, and silica. Each of these may be used alone or two or more of these may be used in combination. Alumina fibers are preferable among the inorganic fibers. The inorganic fibers may be biosoluble fibers.

Furthermore, balloons that are fine hollow spheres including oxide-based ceramics, spherical acrylic particles, graphite, or the like may be added to the adhesive paste, if necessary. The balloons are not particularly limited, and examples thereof include alumina balloon, glass micro balloon, shirasu balloon, fly ash balloon (FA balloon), mullite balloon, and the like.

(5) Next, the honeycomb aggregate body is heated to solidify the adhesive paste, whereby a rectangular pillar-shaped ceramic block is manufactured.

The heating and solidifying of the adhesive paste may be performed under conditions that bans been conventionally employed for manufacturing honeycomb filters.

(6) The ceramic block is subjected to cutting (cutting process).

Specifically, the periphery of the ceramic block is cut with a diamond cutter, whereby a ceramic block whose periphery is cut into a substantially round pillar shape is manufactured.

(7) A peripheral coating material paste is applied to the peripheral face of the substantially round pillar-shaped ceramic block, and is dried and solidified to form a periphery coat layer (periphery coat layer forming process).

The adhesive paste may be used as the peripheral coating material paste. Alternatively, the peripheral coating material paste may be a paste having a composition different from that of the adhesive paste.

The periphery coat layer is not necessarily formed and may be formed, if necessary.

The peripheral shape of the ceramic block is adjusted by the periphery coat layer, and thereby a round pillar-shaped honeycomb filter is obtained.

The honeycomb filter including the honeycomb fired bodies can be manufactured through the aforementioned processes.

Although the honeycomb filter having a predetermined shape is manufactured by cutting, the honeycomb filter may also be allowed to have a predetermined shape such as round-pillar shape as follows: honeycomb fired bodies of a plurality of shapes, each having an outer wall on the periphery thereof, are manufactured in the honeycomb fired body manufacturing process, and then the honeycomb fired bodies of a plurality of shapes are combined with one another with the adhesive layer residing therebetween. In this case, the cutting process can be omitted.

Hereinafter, the effects of the honeycomb filter according to the first embodiment of the present invention are listed.

(1) In the honeycomb filter according to the present embodiment, the exhaust gas introduction cells include the first exhaust gas introduction cells and the second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells; each exhaust gas emission cell has a cross sectional area that is equal to or larger than the cross sectional area of each second exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells, and each first exhaust gas introduction cell has a cross sectional area of 0.7 mm² or smaller in a direction perpendicular to the longitudinal direction of the cells. Consequently, the opening of each first exhaust gas introduction cell becomes small as the amount of PMs accumulated in the honeycomb filter reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. At the same time, as described below, the passage resistance upon passage of exhaust gas through the exhaust gas emission cells as well as the outflow resistance upon emission of exhaust gas from the honeycomb filter are small because the cross sectional area of each exhaust gas emission cell is large; and PMs accumulate widely and thinly in the first exhaust gas introduction cells and the second exhaust gas introduction cells because the cross sectional area of each second exhaust gas introduction cell is larger than the cross sectional area of each first exhaust gas introduction cell. Thus, the pressure can be maintained small until the amount of accumulated PMs reaches a level where the pressure loss rapidly increases (i.e., until the amount of accumulated PM is 6 to 10 g/L).

(2) In the honeycomb filter according to the present embodiment, the cross sectional area of each first exhaust gas introduction cell may be 40 to 70% the size of the cross sectional area of each second exhaust gas introduction cell.

This setting of the cross sectional area ratio of the first exhaust gas introduction cell and the second exhaust gas introduction cell can provide difference between the resistance caused upon flowing of exhaust gas through the first exhaust gas introduction cells and the resistance caused upon flowing of exhaust gas through the second exhaust gas introduction cells, thereby enabling effective reduction of the pressure loss.

(3) In the honeycomb filter according to the present embodiment, the ratio of the length of the side of the second exhaust gas introduction cell facing the exhaust gas emission cell to the length of the side of the first exhaust gas introduction cell facing the exhaust gas emission cell may be not more than 0.8.

The aforementioned ratio of the length of the side of the first exhaust gas introduction cell to the length of the side of the second exhaust gas introduction cell enables easier passage of exhaust gas through the cell walls separating the exhaust gas emission cells and the first exhaust gas introduction cells, effective suppression of the initial pressure loss, and prevention of an increase in the rate of increase of the pressure loss until the amount of accumulated PMs reaches 6 to 10 g/L after accumulation of PMs.

(4) In the honeycomb filter according to the present embodiment, the material of the honeycomb fired bodies may include silicon carbide or silicon-containing silicon carbide. Such a material enables to provide a honeycomb filter having excellent heat resistance.

(5) In the honeycomb filter according to the present embodiment, the thickness of the cell walls separating the cells may be uniform at any part.

This setting for the entire thickness of the cell walls enables to provide a honeycomb filter having the same effects at any part thereof.

(6) In the honeycomb filter according to the present embodiment, the thickness of the cell walls may be 0.10 to 0.46 mm.

The cell walls having the aforementioned thickness are sufficient for capturing PMs in exhaust gas, and also enable efficient suppression of increase in the pressure loss.

(7) In the honeycomb filter according to the present embodiment, the average pore diameter of pores in the cell walls may be 8 to 25 μm in the honeycomb fired body forming the honeycomb filter.

Owing to the above average pore diameter of pores in the cell walls, it is possible to capture PMs at a high capturing efficiency while suppressing an increase in the pressure loss.

(8) In the honeycomb filter according to the present embodiment, the exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side.

Thus, the entire honeycomb filter can exert the same or similar effects so that disadvantages caused by local shape variations in the honeycomb filter can be prevented.

(9) In the honeycomb filter according to the present embodiment, cracks tend not to be caused by thermal shock that occurs upon burn-removal (regeneration) of accumulated PMs in the honeycomb filter.

In the honeycomb filter according to the present embodiment, the plugged portions provided at the end at the exhaust gas emission side, namely the plugged portions in the first exhaust gas introduction cells and the second exhaust gas introduction cells are present in vertical and horizontal rows. Each plugged portion has a width equal to or larger than the length of one side of the first exhaust gas introduction cell. In ordinary regeneration of honeycomb filters, heat burns firstly PMs accumulated at the exhaust gas introduction side of a honeycomb filter. Then, burning of PMs is transferred by exhaust gas flow to the emission side of the honeycomb filter so that all the PMs are burned. For this reason, positions closer to the emission side of the honeycomb filter are exposed to higher temperatures, and easily have temperature difference along the diameter direction of the honeycomb filter. Thus, cracks occur due to the thermal stress. Such cracks are prominent in the case where the cross sectional areas are different among the cells, and also in the case where the cross sections of the exhaust gas introduction cells where PMs accumulate have square cross section(s). In contrast, in the honeycomb filter according to the embodiment of the present invention, the plugged portions aligned in vertical and horizontal rows at the exhaust gas emission side function as thermally conductive layers and as layers for dissipating heat to outside. Thus, the temperature difference is small along the diameter direction at the end of the exhaust gas emission side of the honeycomb filter. This reduces the thermal stress to prevent occurrence of cracks. Moreover, in the honeycomb filter according to the embodiment of the present invention, the amount of the plug in the second exhaust gas introduction cells, in which a larger amount of PMs accumulate compared with the first exhaust gas introduction cells, is large due to the size of the cross sectional area, and thereby the thermal capacity of the plugged portions increase. Such plugged portions can prevent an increase in the temperature of the second exhaust gas introduction cells, where the heat of burning PMs is higher, even in consideration of each cell. This reduces the temperature difference in the diameter direction of the honeycomb filter, thereby presumably reducing the thermal stress to be caused.

Thus, the honeycomb filter according to the present embodiment can prevent cracks during the regeneration even though it includes exhaust gas introduction cells having square cross sections.

(10) In the honeycomb filter according to the present embodiment, the outer wall enables to not only enhance the strength of the honeycomb fired body but also further reduce partial variations in the volume ratio in the exhaust gas emission cells and exhaust gas introduction cells in the honeycomb fired body, allowing the exhaust gas to flow more uniformly. As a result, the pressure loss can be reduced.

Hereinafter, examples are given for more specifically describing the first embodiment of the present invention. However, the present invention is not limited only to the examples.

Example 1

A mixture was obtained by mixing 40.6% by weight of a silicon carbide coarse powder having an average particle size of 22 μm, 17.4% by weight of a silicon carbide fine powder having an average particle size of 0.5 μm, and 10.6% by weight of acrylic resin baaing an average particle size of 40 μm as a pore-forming agent. To the mixture were added 6.4% by weight of an organic binder (methylcellulose), 3.0% by weight of a lubricant (UNILUB, manufactured by NOF Corporation), 1.4% by weight of glycerin, 20.6% by weight of water and then kneaded to prepare a wet mixture. Thereafter, the wet mixture was extrusion-molded (molding process).

This process provides a raw honeycomb molded body which had the same shape as that of the honeycomb fired body 10 shown in FIG. 4A and in which the cells were not plugged. Herein, the size of each sell was set such that the cross sectional area of each first exhaust gas introduction cell 12 was 0.7 mm² or smaller.

Next, the raw honeycomb molded body was dried using a microwave drying apparatus to obtain dried honeycomb molded bodies. Then, predetermined cells of the dried honeycomb molded body were plugged by filling the cells with a plug material paste.

Specifically, the cells are plugged in a manner that the end at the exhaust gas introduction side and the end at the exhaust gas emission side are plugged at the positions shown in FIG. 4A.

The wet mixture was used as the plug material paste. Thereafter, the dried honeycomb molded body, which has predetermined cells filled with the plug material paste, was dried with a drying apparatus again.

Subsequently, the dried honeycomb molded bodies after plugging of cells were degreased at 400° C. (degrease treatment) and then fired at 2200° C. (firing treatment) under normal pressure argon atmosphere tor three hours.

In this manner, a rectangular pillar-shaped honeycomb fired body was manufactured.

The measurement mentioned below of the length of sides and the cross sectional area can be performed by the aforementioned image analysis of an electron microscope photograph and by use of the aforementioned grain size distribution measurement software (Mac-View (Version 3.5), produced by Mountech Co. Ltd.).

The honeycomb fired body was the honeycomb fired body 10 shown in FIGS. 4A and 4B formed of a silicon carbide sintered body having a porosity of 45%, an average pore diameter of 15 μm, a size of 34.3 mm×34.3 mm×152 mm, the number of cells (cell density) of 290 pcs/inch², a thickness of cell walls of 0.25 mm, and a thickness of the plug of 3 mm.

The exhaust gas emission cell 11 was adjacently surrounded fully by the first exhaust gas introduction cells 12 and 12A and the second exhaust gas introduction cells 14 in the cross section perpendicular to the longitudinal direction of the honeycomb fired body 10.

The first exhaust gas introduction cells 12 and 12A had square cross sections, and the length of sides forming the cross sections of the first exhaust gas introduction cells 12 and 12A was 0.81 mm, and the cross sectional area of each of these cells was 0.66 mm².

The second exhaust gas introduction cells 14 had octagonal cross sections. The length of the hypotenuse side of the second exhaust gas introduction cells facing the exhaust gas emission cell 11 was 50 nm, and the vertical or horizontal sides not facing the exhaust gas emission cell 11 were 0.96 mm.

In other words, the length of the side facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14 was 0.53 as long as the side facing the exhaust gas emission cell 11 among the sides forming the first exhaust gas introduction cell 12.

In the exhaust gas emission cells 11B at the four corners, the length of sides adjacent to the outer wall 17 was 1.24 mm, the length of the vertical or horizontal sides was 0.88 mm, the length of the hypotenuse sides was 0.50 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas emission cells 11A, the length of the side adjacent to the outer wall 17 was 1.67 mm, the length of the vertical side parallel to the side adjacent to the outer wall 17 was 0.96 mm, the length of the horizontal side connected at a right angle to the side adjacent to the outer wall 17 was 0.88 mm, the length of the hypotenuse side was 0.50 mm, and the cross sectional area was 1.95 mm².

The exhaust gas emission cell 11 had an octagonal cross section, and the shape of the cross section was the same as that of the second exhaust gas introduction cell 14. The length of the hypotenuse side facing the second exhaust gas introduction cell 14 was 0.50 mm, and the vertical or horizontal sides facing the first exhaust gas introduction cells 12 was 0.96 mm.

The thickness of the outer wall 17 was 0.40 mm

The cross sectional area of the first exhaust gas introduction cell 12 was 0.66 mm², the cross sectional areas of the second exhaust gas introduction cell 14 and the exhaust gas emission cell 11 were both 2.54 mm². In other words, the cross sectional area of the first exhaust gas introduction cell 12 was 26.0% the size of the cross sectional area of the second exhaust gas introduction cell 14.

Moreover, the cross sectional area of the exhaust gas emission cell 11 was equal in size to the cross sectional area of the second exhaust gas introduction cell 14, and was larger than the cross sectional area of the first exhaust gas introduction cell 12.

The honeycomb fired body was in a rectangular pillar shape that was chamfered so that the corner portions at end faces thereof were formed by curved lines.

Next, a plurality of the honeycomb fired bodies were combined using an adhesive paste containing 30% by weight of alumina fibers having an average fiber length of 20 μm, 21% by weight of silicon carbide particles having an average particle size of 0.6 μm, 15% by weight of silica sol, 5.6% by weight of carboxymethyl cellulose, and 28.4% by weight of water. Subsequently, the adhesive layer was dried and solidified at 120° C. to form an adhesive layer, and thereby a pillar-shaped ceramic block was manufactured.

The periphery of the rectangular pillar-shaped ceramic block was cut out using a diamond cutter to manufacture a substantially round pillar-shaped ceramic block.

Subsequently, a sealing material paste having the same composition as that of the adhesive paste was applied to the peripheral face of the ceramic block. The sealing material paste was dried and solidified at 120° C. to form a periphery coat layer. In this manner a round pillar-shaped honeycomb filter was manufactured.

The diameter of the honeycomb filter was 143.8 mm, and the length in the longitudinal direction was 152 mm.

Comparative Example 1

A raw honeycomb molded body was obtained in the same molding process as in Example 1. Subsequently the raw honeycomb molded body was dried using a microwave drying apparatus to manufacture a dried honeycomb molded body. Then, predetermined cells of the dried honeycomb molded body were plugged by filling the cells with a plug material paste.

The positions of the cells to be plugged were changed from those in Example 1 as follows. The octagonal cells were all plugged in the end face corresponding to the end at the exhaust gas emission side, and the square cells were all plugged in the end face corresponding to the end at the exhaust gas introduction side so that the cells were alternately plugged in both of the end faces.

Consequently, a honeycomb molded body was obtained in which the end at the exhaust gas introduction side and the end at the exhaust gas emission side were plugged at the positions shown in FIG. 19B.

A honeycomb fired body 130 shown in FIGS. 19A and 19B were manufactured through the same processes as in Example 1, and a honeycomb filter 120 was manufactured.

In the cross section of the honeycomb fired body 130 in the direction perpendicular to the longitudinal direction of the cells, all the exhaust gas introduction cells 132 had an octagonal cross section except for the exhaust gas introduction cells 132A and 132B which were adjacent to the outer wall 137.

The sides facing the exhaust gas emission cell 131 were vertical or horizontal sides having a length of 0.96 mm.

The sides facing other exhaust gas introduction cells 132, 132A, and 132B were hypotenuse sides having a length of 0.50 mm.

All the exhaust gas emission cells 131 and 131A had a square cross section. The length of the sides forming the cross sections of the exhaust gas emission cells 131 and 131A was 0.81 mm.

In the exhaust gas introduction cells 132B at the four corners, the length of sides adjacent to the outer wall 137 was 1.24 mm, the length of the vertical or horizontal sides was 0.88 mm, the length of the hypotenuse sides was 0.50 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas introduction cells 132A, the length of the side adjacent to the outer wall 137 was 1.67 mm, the length of the vertical side parallel to the side adjacent to the outer wall 17 was 0.96 mm, the length of the horizontal line connected at a right angle to the side adjacent to the outer wall 17 was 0.88 mm, the length of the hypotenuse side was 0.50 mm, and the cross sectional area was 1.95 mm².

The thickness of the cell walls 133 was 0.25 mm, and the thickness of the outer walls was 0.40 mm.

The cross sectional area of the exhaust gas introduction cell 132 was 2.54 mm², and the cross sectional area of the exhaust gas emission cell 131 was 0.66 mm². In other words, the cross sectional area of the exhaust gas introduction cell 132 was larger than the cross sectional area of the exhaust gas emission cell 131.

The honeycomb filters manufactured in Example 1 and Comparative Example 1 were measured for a relation between the amount of captured PMs and the pressure loss using a pressure loss measuring apparatus as shown in FIG. 7.

FIG. 7 is an explanatory view schematically illustrating a method for measuring the pressure loss through actual operation of the diesel engine.

The pressure loss measuring apparatus 310 has the following structure: the honeycomb filter 20 fixed inside a metal casing 313 is disposed in an exhaust gas tube 312 of a 1.6-liter diesel engine 311, and a pressure gauge 314 is attached in a manner such that it can detect the pressure in front and back of the honeycomb filter 20.

The honeycomb filter 20 is disposed such that the end at the exhaust gas introduction side is closer to the exhaust gas tube 312 of the diesel engine 311. Namely, the honeycomb filter 20 is disposed to allow exhaust gas to flow in the cells which are open at the exhaust gas introduction side end.

The diesel engine 311 was operated with the number of rotation of 3100 rpm and a torque of 50 Nm to allow exhaust gas from the diesel engine 311 to flow into the honeycomb filter 20 so that PMs were captured by the honeycomb filter.

Then, a relation was determined between the amount (g/L) of captured PMs per liter of an apparent volume of the honeycomb filter and the pressure loss (kPa).

FIG. 8 is a graph showing the relation between the PM capture amount and the pressure loss in the honeycomb filter, measured in Example 1 and Comparative Example 1.

As is clear from the graph in FIG. 8, compared to the honeycomb filter in Comparative Example 1, the honeycomb filter in Example 1 shows that the pressure loss is small in the normal usage range (i.e., until the amount of captures PMs reaches 6 g/L), and the value of the pressure loss rapidly increases around when the PM capture amount exceeds 6 g/L. This notifies the user of the need to carry out the regeneration treatment ahead of time, and thus the occurrence of the excessive capture is suppressed.

Second Embodiment

The following will describe a honeycomb filter according to the second embodiment of the present invention.

The honeycomb filter according to the second embodiment includes exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side, and exhaust gas introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells, and the cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cell is 0.7 mm² or smaller.

In addition, the exhaust gas emission cells are each adjacently surrounded fully by the first exhaust gas introduction cells and the second exhaust gas introduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of the cells, each second exhaust gas introduction cell has a larger cross sectional area than each first exhaust gas introduction cell, and each exhaust gas emission cell has the same cross sectional area as each second exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells and the exhaust gas introduction cells are each polygonal, and a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the first exhaust gas introduction cell is longer than a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the second exhaust gas introduction cell.

The exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion, in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side.

Namely, the honeycomb filter according to the second embodiment has the same structure as that of the honeycomb filter according to the first embodiment except that the honeycomb filter according to the second embodiment is formed of a single honeycomb fired body having an outer wall on the periphery thereof. Such a honeycomb filter formed of a single honeycomb fired body is referred to also as an integrated honeycomb filter.

FIG. 9A is a perspective view schematically illustrating one example of the integrated honeycomb filter according to the second embodiment of the present invention. FIG. 9B is a B-B line cross sectional view of the integrated honeycomb filter.

In the honeycomb filter 30 shown in FIG. 9A and FIG. 9B, exhaust gas emission cells 31 hawing an octagonal cross section are each adjacently surrounded fully by first exhaust gas introduction cells 32 each having a square cross section and second exhaust gas introduction cells 34 each having an octagonal cross section across porous cell walls therebetween. The first exhaust gas introduction cells 32 and the second exhaust gas introduction cells 34 are alternately arranged around each exhaust gas emission cell 31. Each second exhaust gas introduction cell 34 has a larger cross section than each first exhaust gas introduction cell 32, and each exhaust gas emission cell 31 has the same cross sectional area as each second exhaust gas introduction cell 34. In addition, the cross sectional area of each first exhaust gas introduction cell 32 is 0.7 mm² or smaller.

The length of a side 32 a facing the exhaust gas emission cell 31 among the sides forming the cross sectional shape of the first exhaust gas introduction cell 32 is longer than a side 34 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 34.

The honeycomb filter 30 according to the present embodiment is formed of a single honeycomb fired body, and has an outer wall 37 on the periphery of the honeycomb fired body. Preferable examples of the material forming the honeycomb filter 30 according to the present embodiment include cordierite and aluminum titanate. These materials have a low coefficient of thermal expansion. Thus, cracks or the like caused by thermal stress during regeneration or the like rarely occur even in a large scale honeycomb filter.

The features of the embodiment other than those mentioned above are the same as the features described in relation to the first embodiment, and thus the explanation thereof is omitted.

The honeycomb filter 30 according to the present embodiment can be manufactured by using the honeycomb fired body manufactured in the first embodiment as it is, or can be manufactured in the same manner as in the first embodiment of the present involution, except that a periphery coat layer is formed on the periphery thereof. This embodiment does not require the processes (4), (5), and (6) in the method for manufacturing the honeycomb filter according to the first embodiment of the present invention. Furthermore, the process (7) is not necessary in the case of not forming the periphery coat layer.

The honeycomb filter 30 according to the present embodiment is substantially the same as the honeycomb filter 20 according to the first embodiment concerning the manners of basic arrangement of cells, shapes, plugging, and the like. Thus, the honeycomb filter 30 according to the present embodiment can exert the same effects as the effects (1) to (3) described in relation to the first embodiment.

Third Embodiment

The following will describe a honeycomb filter according to the third embodiment of the present invention.

The honeycomb filter according to the third embodiment of the present invention includes a plurality of honeycomb fired bodies each having an outer wall on the periphery thereof. Each honeycomb fired body includes exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side, and exhaust gas introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells. The honeycomb fired bodies are combined with one another by adhesive layers residing therebetween. The cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller.

The exhaust gas emission cells are each adjacently surrounded fully by the first exhaust gas introduction cells and the second exhaust gas introduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of the cells, each second exhaust gas introduction cell has a larger cross sectional area than each first exhaust gas introduction cell, and each exhaust gas emission cell has the same cross sectional area as each second exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells and the exhaust gas introduction cells are each polygonal, and a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the first exhaust gas introduction cell is longer than a side facing one exhaust gas emission cell among the sides forming the cross sectional shape of the second exhaust gas introduction cell.

In regard to the cells adjacent to the outer wall, the exhaust gas emission cells and the first exhaust introduction cells are alternately arranged.

The exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion in a direction perpendicular to the longitudinal direction of the cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side.

Namely, the honeycomb filter according to the third embodiment has the same structure as that of the honeycomb filter according to the first embodiment. They are the same in terms of the basic shape and arrangement of the cells but different in that, in the honeycomb filter according to the third embodiment, the cross sectional shape of the cells adjacent to the outer wall is the same as the cross sectional shape of the cells other than the cells adjacent to the enter wall.

FIG. 10A is a perspective view schematically illustrating one example of the honeycomb filter according to the third embodiment of the present invention. FIG. 10B is a perspective view schematically illustrating the honeycomb fired body forming the honeycomb filter.

In a honeycomb fired body 10 a forming a honeycomb filter 30 shown in FIG. 10A and FIG. 10B, the cross sectional shape and arrangement of the cell walls are essentially the same as the cross sectional shape and arrangement of the cell walls of the honeycomb fired body 10 forming the honeycomb filter 20 shown in FIGS. 4A and 4B. The exhaust gas emission cells 11 having an octagonal cross section are each adjacently surrounded fully by the first exhaust gas introduction cells 12 each having a square cross section and the second exhaust gas introduction cells 14 each having an octagonal cross section across porous cell walls therebetween. The first exhaust gas introduction cells 12 and the second exhaust gas introduction cells 14 are alternately arranged around each exhaust gas emission cell 11. Each second exhaust gas introduction cell 14 has a larger cross sectional area than each first exhaust gas introduction cell 12, and each exhaust gas emission cell 11 has the same cross sectional area as each second exhaust gas introduction cell 14. An outer wall 17 a is formed on the periphery of the honeycomb fired body 10. The cells adjacent to the outer wall 17 a include the first exhaust gas introduction cells 12 and the exhaust gas emission cells 11. The cross sectional area of each first exhaust gas introduction cell is 0.7 mm² or smaller.

The length of the side 12 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the first exhaust gas introduction cell 12 is longer than the side 14 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14.

The honeycomb fired body 10 a is different from the honeycomb fired body 10 included in the honeycomb filter 20 shown in FIGS. 4A and 4B in that, as shown in FIG. 10B, the exhaust gas emission cells 11 and the first exhaust gas introduction cells 12 adjacent to the outer wall have the same cross sectional shapes as the exhaust gas emission cells 11 and the first exhaust gas introduction cells 12 not adjacent to the outer wall, respectively.

Next, the following will describe a modified example of the honeycomb filter according to the third embodiment of the present invention.

FIG. 11A is a perspective view schematically illustrating one modified example of a honeycomb fired body forming the honeycomb filter according to the third embodiment of the present invention. FIG. 11B is an end face view of the honeycomb fired body in FIG. 11A.

In a honeycomb fired body 10 b shown in FIG. 11A and 11B, all the first exhaust gas introduction cells 12 have the same shape and all the second exhaust gas introduction cells 14 have the same shape. A side 170 b defining an outer periphery of an outer wall 17 b changes in accordance with the shapes of the first exhaust gas introduction cells 12 and the exhaust gas emission cells 11 adjacent to the outer wall 17 b, and the thickness of the outer wall 17 b is uniform.

In other words, to make the thickness of the outer wall 17 b uniform, the side 170 b defining the outer periphery of the outer wall 17 b becomes rugged along with the shapes of the first exhaust gas introduction cells 12 and the exhaust gas emission cells 11 adjacent to the outer wall 17 b.

FIG. 11C is an end face view illustrating another modified example of a honeycomb fired body forming the honeycomb filter according to the third embodiment of the present invention.

In a honeycomb fired, body 10 c shown in FIG. 11C, all the cells adjacent to an outer wall 17 c are exhaust gas introduction cells. The shape of a second exhaust gas introduction cell 14A is partially deformed to be hexagonal in accordance with the line along the inner wall, which forms the outer wall 17 c, of the first exhaust gas introduction cells 12A adjacent to the outer wall 17 c, as compared to the shape of the exhaust gas introduction cells not adjacent to the outer wall 17 c. In addition, an exhaust gas emission cell 14B at each corner portion is also partially deformed to be pentagonal.

As described above, all the cells adjacent to the outer wall may be exhaust gas introduction cells with an aim of enhancing the aperture ratio of the exhaust gas introduction cells.

The features of the embodiment other than those mentioned above are the same as the features described in relation to the first embodiment, and thus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can be manufactured by the same method as described in the first embodiment of the present invention, except that the shape of the mold used in the extrusion molding is changed.

The honeycomb filter according to the present embodiment is substantially the same as the honeycomb filter 20 according to the first embodiment concerning the manners of basic arrangement of cells, shapes, plugging, and the like. Thus, the honeycomb filter 30 according to the present embodiment can exert the same effects as the effects (1) to (10) described in relation to the first embodiment.

Fourth Embodiment

The following will describe a honeycomb filter according to the fourth embodiment of the present invention. Features not described below are substantially the same as those in the honeycomb filter according to the first embodiment.

The honeycomb filter according to the fourth embodiment of the present invention includes a plurality of honeycomb fired bodies each having an outer wall on the periphery thereof. Each honeycomb fired body includes exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side, and exhaust gas introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells. The honeycomb fired bodies are combined with one another by adhesive layers residing therebetween, and the cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller.

The exhaust gas emission cells are each adjacently surrounded fully by the first exhaust gas introduction cells and the second exhaust gas introduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of the cells, each second exhaust gas introduction cell has a larger cross sectional area than each first exhaust gas introduction cell, and each exhaust gas emission cell has the same cross sectional area as each second exhaust gas introduction cell.

In regard to the cross section perpendicular to the longitudinal direction of the cells, the cross sectional shapes of the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells each hare a square cross sectional shape, and one of the sides forming the cross sectional shape of the first exhaust gas introduction cell faces one exhaust gas emission cell, and none of the sides forming the cross sectional shape of the second exhaust gas introduction cell faces the sides forming the exhaust gas emission cell.

The cells adjacent to the outer wall include the first exhaust gas introduction cells and the exhaust gas emission cells.

In regard to the cross sectional shape in a direction perpendicular to the longitudinal direction of the cell, the exhaust gas introduction cells and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side.

In other words, the honeycomb filter according to the fourth embodiment has substantially the same structure as the honeycomb filter according to the first embodiment, except that all the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells each have a square cross section, and also except for the features mentioned below.

FIG. 12A is an end face view schematically illustrating one example of the cell arrangement at an end face of the honeycomb fired body forming the honeycomb filter according to the fourth embodiment of the present invention.

In a honeycomb fired body 40 forming the honeycomb filter shown in FIG. 12A, the exhaust gas emission cells 41 having a square cross section are each adjacently surrounded fully by first exhaust gas introduction cells 42 each having a square cross section and second exhaust gas introduction cells 44 each having a square cross section across porous cell walls therebetween. The first exhaust gas introduction cells 42 and the second exhaust gas introduction cells 44 are alternately arranged around each exhaust gas emission cell 41. Each second exhaust gas introduction cell 44 has a larger cross sectional area than each first exhaust gas introduction cell 42, and each exhaust gas emission cell 41 has the same cross sectional area as each second exhaust gas introduction cell 44.

The cross sectional area of each first exhaust gas introduction cell is 0.7 mm² or smaller.

In the cross section of the three kinds of adjacent cells, namely, the exhaust gas emission cell 41, the second exhaust gas introduction cell 44, and the first exhaust gas introduction cells 42, a side 41 a facing the first exhaust gas introduction cell 42 across a cell wall 43 among the sides of the square exhaust gas emission cell 41 is parallel to a side 42 a facing the exhaust gas emission cell 41 across the cell wall 43 among the sides of the square first exhaust gas introduction cell 42.

Moreover, a side 42 b facing the second exhaust gas introduction cell 44 across the cell wall 43 among the sides of the first exhaust gas introduction cell 42 is parallel to a side 44 b facing the first exhaust gas introduction cell 12 across the cell wall 43 among the sides of the second exhaust gas introduction cell 44. Furthermore, the distances between the parallel sides of all the above pairs are the same. That is, the distance between the parallel sides 41 a and 42 a, and the distance between the parallel sides 42 b and 44 b, are the same.

The square exhaust gas emission cell 41 is adjacently surrounded by alternately arranged four pieces of the first square exhaust gas introduction cells 42 and four pieces of the second square exhaust gas introduction cells 44 across the cell walls 43. The cross sectional area of the second exhaust gas introduction cell 44 is larger than the cross sectional area of the first exhaust gas introduction cell 42.

Furthermore, the exhaust gas emission cells 41, the first exhaust gas introduction cells 42, and the second exhaust gas introduction cells 44 are each arranged in a manner satisfying the conditions below.

Namely, among hypothetical segments connecting geometric centers of gravity of the square shapes of the four second exhaust gas introduction cells 44 surrounding the exhaust gas emission cell 41, an intersection of the two segments crossing the square shape region of the exhaust gas emission cell 41 is identical with the geometric center of gravity of the square cross section of the exhaust gas emission cell 41.

Moreover, among the hypothetical segments connecting the geometric centers of gravity of the square shapes of the four second exhaust gas introduction cells 44, the four segments not crossing the square shape region of the exhaust gas emission cell 41 forms a square, and midpoints of the respective sides of the square are identical with the geometric centers of gravity of the respective square shapes of the four first exhaust gas introduction cells 42 surrounding the exhaust gas emission cell 41.

As described above, the square exhaust gas emission cell 41 is adjacently surrounded by alternately arranged four pieces of the first square exhaust gas introduction cells 42 and four pieces of the second square exhaust gas introduction cells 44 across the cell walls 43 to form a single unit. The unit is two-dimensionally repeated, where the first exhaust gas introduction cells 42 and the second exhaust gas introduction cells 44 in the unit are shared between adjacent cell units, to form a honeycomb filter. The units share the first exhaust gas introduction cells 42 and the second exhaust gas introduction cells 44. Thus, the first exhaust gas introduction cell 42 and the second exhaust gas introduction cell 44, which face the exhaust gas emission cell 41 across the cell walls 43, face the exhaust gas emission cell 41 in the adjacent unit across the cell wall 43.

FIG. 13 is an enlarged cross sectional view perpendicular to the longitudinal direction of the honeycomb filter. FIG. 13 illustrates how each cell unit (cell structure) is two-dimensionally, i.e. in X and Y directions shown in FIG. 13, repeated in the case where the first exhaust gas introduction cells 42, the second exhaust gas introduction cells 44, and the exhaust gas emission cells 41 are square and the aforementioned conditions are satisfied, and also illustrates how the first exhaust gas introduction cells 42 and the second exhaust gas introduction cells 44 in the unit are shared between the cell units (cell structure). A cell unit 1, a cell unit 2, and a cell unit 3 each have a structure in which the exhaust gas emission cell 41 is fully surrounded by alternately arranged four pieces of the first exhaust gas introduction cells 42 and four pieces of the second exhaust gas introduction cells 44 across the cell walls 43 in a manner satisfying the aforementioned conditions.

The cell unit 2 has the same structure as that of the cell unit 1. The cell unit 2 is adjacent to the cell unit 1 in the X direction while sharing one piece of the first exhaust gas introduction cell 42 and two pieces of the second exhaust gas introduction cells 44 with the cell unit 1. The cells shared between the cell unit 1 and the cell unit 2 are depicted as “shared portion 2” in FIG. 13. The cell unit 3 has the same structure as that of the cell unit 1. The cell unit 3 is adjacent to the cell unit 1 in the Y direction while sharing one piece of the first exhaust gas introduction cell 42 and two pieces of the second exhaust gas introduction cells 44 with the cell unit 1. The cells shared between the cell unit 1 and the cell unit 3 are depicted as “shared portion 1”in FIG. 13.

Meanwhile, FIG. 13 shows four segments h, i, j, and k that do not cross the square shape region of the exhaust gas emission cell 41, and hypothetical two augments l and m that cross the square shape region of the exhaust gas emission cell 41, among hypothetical segments connecting the geometric centers of gravity of the square shapes of the four pieces of the second exhaust gas introduction cells 44. The “shared portion 2” is depicted by cross-hatching with segments in the same direction as that of the segment m, and the “shared portion 1” is depicted by cross-hatching with segments in the same direction as that of the segment l.

As shown in FIG. 13, an intersection of the two segments l and m is identical with the geometric center of gratuity of the exhaust gas emission cell 41.

In the cross section of the cells, one of sides forming the cross sectional shape of the first exhaust gas introduction cell 42 faces one exhaust gas emission cell 41. Also, the second exhaust gas introduction cell 44 and the exhaust gas emission cell 11 are arranged so that they face each other at their corner portions. Thus, none of the sides forming the cross sectional shape of the second exhaust gas introduction cell 44 faces the sides forming the exhaust gas emission cell 41. The cells adjacent to she outer wall 47 include the first exhaust gas introduction cells 42 and the exhaust gas emission cells 41.

None of the sides forming the cross sectional shape of the second exhaust gas introduction cells faces exhaust gas emission cells in the present embodiment, and thus exhaust gas more easily flows into the first exhaust gas introduction cells at an initial stage as compared to the first embodiment. For this reason, PMs accumulate earlier on the inner cell walls of the first exhaust gas introduction cells corresponding to the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells. Consequently, the aforementioned switching of the main channel occurs further earlier. Therefore, PMs tend to accumulate uniformly and thinly on the inner cell walls of the first exhaust gas introduction cells and the inner cell walls of the second exhaust gas introduction cells so that the pressure loss can be further reduced after accumulation of a certain amount of PMs until the amount of accumulated PMs reaches 6 to 10 g/L. At the same time, the cross section area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller, so that the openings of the first exhaust gas introduction cells become small as the amount of accumulated PM reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. Consequently, the regeneration treatment can be reliably carried out at an appropriate time. Thus, in the honeycomb filter according to the present embodiment, the excessive capture and the increase in the amount of heat generated by regeneration are suppressed, and the honeycomb filter is thus less likely to have cracks and dissolution loss.

In the honeycomb filter according to the present embodiment, the plugged portions provided at the end at the exhaust gas emission side, namely the plugged portions in the first exhaust gas introduction cells and the second exhaust gas introduction cells are present in vertical and horizontal rows. Each plugged portion has a width equal to or larger than the length of one side of the first exhaust gas introduction cell. In ordinary regeneration of honeycomb filters, heat burns firstly PMs accumulated at the exhaust gas introduction side of a honeycomb filter. Then, burning of PMs is transferred by exhaust gas flow to the emission side of the honeycomb filter so that all the PMs are burned. For this reason, positions closer to the emission side of the honeycomb filter are exposed to higher temperatures, and easily have temperature difference along the diameter direction of the honeycomb filter. Thus, cracks occur due to the thermal stress. Such cracks are prominent in the case where the cross sectional areas are different among the cells, and also in the case where the cross sections of the exhaust gas introduction cells where PMs accumulate have square cross section(s). In contrast, in the honeycomb filter according to the embodiment of the present invention, the plugged portions aligned in vertical and horizontal rows at the exhaust gas emission side function as thermally conductive layers and as layers for dissipating heat to outside. Thus, the temperature difference is small along the diameter direction of the honeycomb filter. This reduces the thermal stress to be caused to prevent occurrence of cracks. Moreover, in the honeycomb filter according to the embodiment of the present invention, the amount of the plug in the second exhaust gas introduction cells, in which a larger amount of PMs accumulate compared with the first exhaust gas introduction cells, is large due to the size of the cross sectional area, and thereby the thermal capacity of the plugged portions increase. Such plugged portions can prevent an increase in the temperature of the second exhaust gas introduction cells, where the heat of burning PMs is higher, even in consideration of each cell. This reduces the temperature difference in the diameter direction of the honeycomb filter, thereby reducing the thermal stress to be caused.

Thus, the honeycomb filter according to the present embodiment can prevent cracks during the regeneration even though it includes exhaust gas introduction cells having square cross sections.

The cross sectional area of each first exhaust gas introduction cell is 0.7 mm² or smaller, and the cross sectional area of each first exhaust gas introduction cell 42 is preferably 40 to 70% and more preferably 50 to 65% the size of the cross sectional area of each second exhaust gas introduction cell 44.

In the honeycomb fired body shown in FIGS. 12A to 12C, the cross sectional area of each exhaust gas emission cell 41 is equal to the cross sectional area of each second exhaust gas introduction cell 44; however, the cross sectional area of each exhaust gas emission cell 41 may be larger than the cross sectional area of each second exhaust gas introduction cell 44.

The cross sectional area of each exhaust gas emission cell 41 is preferably 1.05 to 1.5 times the size of the cross sectional area of each second exhaust gas introduction cell 44.

The following will describe the thickness of the cell walls in the cross section of the honeycomb fired body 40 according to the fourth embodiment based on the aforementioned definition of the cell walls. Supposing that a straight line Z₄₂ connecting the center of gravity O₄₁ of the exhaust gas emission cell 41 and the center of gravity O₄₂ of the first exhaust gas introduction cell 42 is given, the thickness of the cell wall 43 at a part overlapped with the straight line Z₄₂ (the thickness between the side 42 a and the side 41 a) is determined as thickness X₁. Supposing that a straight line Z₄₄ connecting the center of gravity O₄₄ of the second exhaust gas introduction cell 44 and the center of gravity O₄₁ of the exhaust gas emission cell 41 is given, the thickness of the cell wall 43, which separates the second exhaust gas introduction cell 44 and the exhaust gas emission cell 41, at a part overlapped with the straight line Z₄₄ (the length between a corner portion 44 c of the second exhaust gas introduction cell 44 and a corner portion 41 c of the exhaust gas emission cell 41) is determined as thickness Y₁.

The thickness of the cell walls of the honeycomb filter 40 varies depending on the parts as shown in FIG. 12A; however, the thicknesses, including the thickness X₁ and the thickness Y₁, may be set to be within a range of 0.10 to 0.46 mm.

Next, the following will describe a modified example of a honeycomb fired body forming the honeycomb filter according to the fourth embodiment of the present invention.

FIG. 12B is an end face view illustrating one modified example of the honeycomb filter according to the fourth embodiment of the present invention.

In a honeycomb fired body 40 a included in the honeycomb filter shown in FIG. 12B, the shape of an exhaust gas emission cell 41A adjacent to an outer wall 47 a is partially deformed to be rectangular in accordance with the line along the inner wall, which forms the outer wall, of the first exhaust gas introduction cells 42A adjacent to the outer wall 47 a, as compared to the shape of an exhaust gas emission cell 41 not adjacent to the outer wall 47 a, An exhaust gas emission cell 41B at each corner portion has a square shape with a smaller cross sectional area compared to the exhaust gas emission cell 41 not adjacent to the outer wall 47 a.

With such shapes of the cells, the boundary between the outer wall 47 a and the exhaust gas emission cells 41A and 41B and the first exhaust gas introduction cells 42A adjacent to the outer wall 47 a is formed linearly, and the thickness of the outer wall 47 a is uniform.

The following will describe another modified example of the honeycomb filter according to the fourth embodiment of the present invention.

FIG. 12C is an end face view illustrating another modified example of the honeycomb fired body forming the honeycomb filter according to the fourth embodiment of the present invention.

In a honeycomb fired body 40 b shown in FIG. 12C, all the first exhaust gas introduction cells 42 have the same shape, and all the exhaust gas emission cells 41 have the same shape. A side 470 b corresponding to an outer periphery of an outer wall 47 b changes in accordance with the shapes of the first exhaust gas introduction cells 42 and the exhaust gas emission cells 41 adjacent to the outer wall 47 b. The thickness of the outer wall 47 b is uniform.

In other words, to make the thickness of the outer wall 47 b uniform, the side 470 b corresponding to the outer periphery of the outer wall 47 b becomes rugged along with the shapes of the first exhaust gas introduction cells 42 and the exhaust gas emission cells 41 adjacent to the outer wall 47 b.

The features of the embodiment other than those mentioned above are the same as the features described in relation to the first embodiment, and thus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can be manufactured in the same manner as in the first embodiment of the present invention, except that a die having a different shape is used in the extrusion molding process.

In the honeycomb fired body 40 according to the present embodiment, unlike the first embodiment, the exhaust gas emission cells 41 and the second exhaust gas introduction cells 44 have square cross sections, and none of the sides forming the cross sectional shape of the second exhaust gas introduction cells 44 faces the exhaust gas emission cells 41. Among the sides forming the cross sectional shape of the first exhaust gas introduction cells 42, a side 42 a faces one of the exhaust gas emission cells 41. Thus, as is the case with the honeycomb filter according to the first embodiment, it is considered that exhaust gas easily flows into the first exhaust gas introduction cells 42 at an initial stage. After accumulation of a certain amount of PMs, exhaust gas tends to easily flow into the second exhaust gas introduction cells 44.

The honeycomb fired body 40 according to the present embodiment is substantially the same as the honeycomb filter 20 according to the first embodiment in terms of basic arrangement of cells, manner of plugging, size difference among the cross sectional areas of the cells, or the like. Thus, the honeycomb filter 40 according to the present embodiment can exert the same effects as the effects (1), (2), (4), and (6) to (10) described in relation to the first embodiment.

Fifth Embodiment

The following will describe a honeycomb filter according to the fifth embodiment of the present invention. Features not described below are substantially the same as those in the honeycomb filter according to the first embodiment.

The honeycomb filter according to the fifth embodiment of the present invention includes a plurality of honeycomb fired bodies each having an outer wall on the periphery thereof. Each honeycomb fired body includes exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side, and exhaust gas introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells. The honeycomb fired bodies are combined with one another by adhesive layers residing therebetween, and the cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller.

In the honeycomb filter according to the fifth embodiment of the present invention, in the cross section perpendicular to the longitudinal direction of the cells, the cross sectional area of each exhaust gas emission cell is equal in size to the cross sectional area of each second exhaust gas introduction cell. The exhaust gas emission cells and the exhaust gas introduction cells are each in a shape formed by a curved line in the cross section perpendicular to the longitudinal direction of the cells, and all the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells have round cross sectional shapes.

The cells adjacent to the outer wall include the first exhaust gas introduction cells and the exhaust gas emission cells.

The honeycomb filter according to the fifth embodiment of the present invention has substantially the same features as the features of the honeycomb filter according to the first embodiment of the present invention, except that the cross sectional shapes of the exhaust gas emission cells, the second exhaust gas introduction cells, and the first exhaust gas introduction cells are different in the direction perpendicular to the longitudinal direction of the cells.

FIG. 14 is an enlarged end face view schematically illustrating one example of the cell arrangement at an end face of a honeycomb fired body forming the honeycomb filter according to the fifth embodiment of the present invention.

A honeycomb fired body 50 according to the fifth embodiment of the present invention includes exhaust gas emission cells 51, first exhaust gas introduction cells 52, cell walls 53, and second exhaust gas introduction cells 54.

The exhaust gas emission cells 51 are each adjacently surrounded fully by the first exhaust gas introduction cells 52 and the second exhaust gas introduction cells 54 across porous cell walls 53 residing therebetween, and the cells adjacent to the outer call 57 include the exhaust gas emission cells 51 and the first exhaust gas introduction cells 52.

In the honeycomb filter shown in FIG. 14, in the cross section perpendicular to the longitudinal direction of the cells, the cross sectional area of each second exhaust gas introduction cell 54 is equal in size to the cross sectional area of each exhaust gas emission cell 51, and the cross sectional area of each first exhaust gas introduction cells 52 is 0.7 mm², which is smaller than the cross sectional area of each second exhaust gas introduction cell 54. The cross sectional area of each first exhaust gas introduction cell 52 is preferably 40 to 70% the size of the cross sectional area of each second exhaust gas introduction cell 54.

All the exhaust gas emission cells 51, the first exhaust gas introduction cells 52, and the second exhaust gas introduction cells 54 have round cross sectional shapes.

The following will describe the thickness of the cell walls in the cross section of the honeycomb fired body 50 forming the honeycomb filter according to the fifth embodiment shown in FIG. 14 based on the aforementioned definition of the cell walls. Supposing that a straight line Z₅₂ connecting the center of gravity O₅₁ of the exhaust gas emission cell 51 and the center of gravity O₅₂ of the first exhaust gas introduction cell 52 is given, the thickness of the cell wall 53 at a part overlapped with the straight line is determined as thickness X₂. Supposing that a straight line Z₅₄ connecting the center of gravity O₅₄ of the second exhaust gas introduction cell 54 and the center of gravity O₅₁ of the exhaust gas emission cell 51 is given, the thickness of the cell wall 53, which separates the second exhaust gas introduction cell 54 and the exhaust gas emission cell 51, at a part overlapped with the straight line Z₅₄ is determined as thickness Y₂.

In the honeycomb fired body 50 forming the honeycomb filter according to the present embodiment, the thickness X₂ of the call wall 53 separating the first exhaust gas introduction cell 52 and the exhaust gas emission cell 51 is smaller than the thickness Y₂ of the cell wall 53 separating second exhaust gas introduction cell 54 and the exhaust gas emission cell 51.

This embodiment provides a shape that can nave a larger difference between the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells and the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells. For this reason, the exhaust gas more easily flows into the first exhaust gas introduction cells at an initial stage as compared to the first embodiment. Thus, PMs accumulate earlier on the inner cell walls of the first exhaust gas introduction cells corresponding to the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells. Consequently, the aforementioned switching of the main channel occurs further earlier. Therefore, PMs tend to accumulate uniformly and thinly on the inner cell walls of the first exhaust gas introduction cells and the inner cell walls of the second exhaust gas introduction cells so that the pressure loss can be suppressed until the amount of accumulated PMs reaches 6 to 10 g/L. At the same time, the cross section area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller, so that the opening of each first exhaust gas introduction cell becomes small as the amount of accumulated PM reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. Consequently, the regeneration treatment can be reliably carried out at an appropriate time. Thus, in the honeycomb filter according to the present embodiment, the excessive capture and the increase in the amount of heat generated by regeneration are suppressed, and the honeycomb filter is thus less likely to have cracks and dissolution loss.

The thickness X₂ of the cell wall 53 separating the first exhaust gas introduction cell 52 and the exhaust gas emission cell 51 is preferably 40 to 75% the thickness Y₂ of the cell wall 53 separating the second exhaust gas introduction cell 54 and the exhaust gas emission cell 51.

In the honeycomb fired body 50 according to the present embodiment, the thickness of the cell wall 53 separating the first exhaust gas introduction cell 52 and the second exhaust gas introduction cell 54 may be determined in the same manner as the thickness of the cell wall 53 separating the first exhaust gas introduction cell 52 and the exhaust gas emission cell 51.

In the honeycomb fired body 50 shown in FIG. 14, the thickness of the cell wall 53 separating the first exhaust gas introduction cell 52 and the second exhaust gas introduction cell 54 is smaller than the thickness of the cell wall 53 separating the first exhaust gas introduction cell 52 and the exhaust gas emission cell 51.

In the honeycomb filter according to the fifth embodiment of the present invention, as shown in FIG. 14, the shape of each first exhaust gas introduction cell adjacent to the outer wall may be the same as that of each first exhaust gas introduction cell not adjacent to the outer wall, and the shape of each exhaust gas emission cell adjacent to the outer wall may be the same as that of each exhaust gas emission cell not adjacent to the outer wall. Alternatively, the exhaust gas emission cell may have a side partially deformed in accordance with the line connecting the outermost points on the inner walls of the first exhaust gas introduction cells adjacent to the outer walls, in a manner that thickness of the outer wall except for the corner portions is substantially uniform.

In the honeycomb filter according to the fifth embodiment of the present invention, the thickness of the outer wall may be uniform in accordance with the shapes of the first exhaust gas introduction cells and the exhaust gas emission cells adjacent to the outer wall, and the shapes of the first exhaust gas introduction cells and the exhaust gas emission cells adjacent to the outer wall may be the same as the exhaust gas emission cells not adjacent to the outer wall, respectively. In other words, in this case, the outer wall is bent in accordance with the shapes of the first exhaust gas introduction cells and the exhaust gas emission cells to maintain the uniform thickness.

The features of the embodiment other than those mentioned above are the same as the features described in relation to the first embodiment, and thus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can be manufactured in the same manner as in the first embodiment of the present invention, except that a die having a different shape is used in the extrusion molding process.

Hereinafter, the effects of the honeycomb filter according to the fifth embodiment of the present invention are listed.

The honeycomb filter described in the first embodiment of the present invention has a feature that the side 12 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the first exhaust gas introduction cell 12 is longer than the side 14 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14. Meanwhile, the honeycomb filter according to the fifth embodiment has a feature that all the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells have round cross sectional shapes, and the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is smeller than the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells. The fifth embodiment is different on the above points from the first embodiment. Other features are substantially the same.

A smaller thickness of the cell walls may lead to easier passage of exhaust gas through the cell walls so that the pressure loss is reduced even in the case where all the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells have round cross sectional shapes. Thus, the length of sides forming cross sections of the cells may be considered to correspond to the thickness of the cell walls separating the cells. Hence, the honeycomb filter according to the fifth embodiment of the present invention can exert the same effects as the effects (1), (2), (4), (6) to (8), and (10) described in relation to the first embodiment.

Sixth Embodiment

The following will describe a honeycomb filter according to the sixth embodiment of the present invention. Features not described below are substantially the same as those in the honeycomb filter according to the first embodiment.

The honeycomb filter according to the sixth embodiment of the present invention includes a plurality of honeycomb fired bodies each having an outer wall on the periphery thereof. Each honeycomb fired body includes exhaust gas emission cells each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side, and exhaust gas introduction cells each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side, the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells each having a larger cross sectional area than each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells. The honeycomb fired bodies are combined with one another by adhesive layers residing therebetween, and the cross sectional area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller.

In the honeycomb filter according to the sixth embodiment of the present invention, in the cross section perpendicular to the longitudinal direction of the cells, the cross sectional area of the exhaust gas emission cells is equal in size to the cross sectional area of the second exhaust gas introduction cells. In the cross section perpendicular to the longitudinal direction of the cells, the exhaust gas emission cells and the exhaust gas introduction cells are each in a shape formed by curved lines; and the exhaust gas emission cells and the second exhaust gas introduction cells each have a convex square cross section formed by four outwardly curved lines, whereas the first exhaust gas introduction cells each have a concave square cross section formed by four inwardly curved lines.

The cells adjacent to the outer wall include the exhaust gas emission cells and the first exhaust gas introduction cells.

The honeycomb filter according to the sixth embodiment of the present invention has substantially the same features as the features of the honeycomb filter according to the first embodiment of the present invention, except that the cross sectional shapes of the exhaust gas emission cells, the second exhaust gas introduction cells, and the first exhaust gas introduction cells are different in the direction perpendicular to the longitudinal direction of the cells.

FIG. 15 is an end face view schematically illustrating one example of the cell arrangement in an end face of a honeycomb fired body forming the honeycomb filter according to the sixth embodiment of the present invention.

In the honeycomb filter according to the sixth embodiment of the present invention, a honeycomb fired body 60 includes exhaust gas emission cells 61, first exhaust gas introduction cells 62, cell walls 63, and second exhaust gas introduction cells 64. The exhaust gas emission cells 61 are each adjacently surrounded fully by the first exhaust gas introduction cells 62 and the second exhaust gas introduction cells 64 across porous cell walls 63 residing therebetween.

In the honeycomb filter 60 shown in FIG. 15, in the cross section perpendicular to the longitudinal direction of the cells, the cross sectional area of each second exhaust gas introduction cell 64 is equal in size to the cross sectional area of each exhaust gas emission cell 61, and the cross sectional area of each first exhaust gas introduction cell 62 is smaller than the cross sectional area of each second exhaust gas introduction cell 64, and the cross sectional area of each first exhaust gas introduction cell is 0.7 mm² or smaller. The cross sectional area of each first exhaust gas introduction cell 62 is preferably 40 to 70% the size or the cross sectional area of each second exhaust gas introduction cell 64.

The exhaust gas emission cells 61 and the second exhaust gas introduction cells 64 each have a convex square cross section formed by four outwardly curved lines.

FIG. 16A is an explanatory view schematically illustrating one example of the convex square cell shape. FIG. 16B is an explanatory view schematically illustrating one example of the concave square cell shape. FIG. 16C is an explanatory view schematically illustrating one example of the concave square shape in which a vertex portion is chamfered. FIG. 16D an explanatory view schematically illustrating one example of the convex square shape in which a vertex portion is chamfered.

FIG. 16A shows a second exhaust gas introduction cell 64 having a convex square cross section, and a square 65 formed by connecting four vertices 64 e of the second exhaust gas introduction cell 64.

In the explanation of the embodiments of the present invention, the convex square refers to a figure that is substantially square saving four curved sides. The sides are curved outwardly from the square formed by connecting the four vertices of the substantially square figure.

FIG. 16A shows that the sides 64 a forming the cross section of the second exhaust gas introduction cell 64 are curved (convex) outwardly from the geometric center of gravity of the convex square toward outside the square 65.

Although FIG. 16A illustrates the cross section of the second exhaust gas introduction cell 64 as an example of the convex square cell shape, the cross section of the exhaust gas emission cell 61 is substantially the same as the cross section of the second exhaust gas introduction cell 64.

The first exhaust gas introduction cells 62 each have a concave square cross section formed by four inwardly curved lines.

FIG. 16B shows the first exhaust gas introduction cell 62 having a concave square cross section, and a square 66 formed by connecting four vertices 62 e of the first exhaust gas introduction cell 62.

In the explanation of the embodiments of the present invention, the concave square refers to a figure that is substantially square having four curved sides. The sides are curved (concaved) inwardly from the square formed by connecting the four vertices of the substantially square figure toward the geometric center of gravity of the concave square.

FIG. 16B shows that the sides 62 a forming the cross section of the first exhaust gas introduction cell 62 are curved (concaved) from the square 66 towards the geometric center of gravity of the concave square.

According to the present embodiment, in regard to the cross section in a direction perpendicular to the longitudinal direction of the cells, the first exhaust gas introduction cells each have an acute angle portion that causes resistance by inhibiting gas flow, whereas the second exhaust gas introduction cells each have obtuse angles that allow easy gas flow. Thus, in comparison to the first embodiment, after only a small amount of PMs accumulate on the inner cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, exhaust gas easily flows into the first exhaust gas introduction cells. Therefore, PMs tend to accumulate uniformly and thinly on the inner walls of the first exhaust gas introduction cells and the inner walls of the second exhaust gas introduction cells, and the pressure loss can be maintained small until the amount of accumulated PMs reaches 6 to 10 g/L. At the same time, the cross section area of each first exhaust gas introduction cell in a direction perpendicular to the longitudinal direction of the cells is 0.7 mm² or smaller, so that the opening of each first exhaust gas introduction cells becomes small as the amount of accumulated PM reaches 6 to 10 g/L, causing a rapid increase in the pressure loss. Consequently, the regeneration treatment can be reliably carried cut at an appropriate time. Thus, in the honeycomb filter according to the present embodiment, the excessive capture and the increase in the amount of heat generated by regeneration are suppressed, and the honeycomb filter is thus less likely to have cracks and dissolution loss.

In the explanation of the embodiments of the present invention, the convex square and the concave square include shapes that are chamfered at the vertex portions thereof.

FIG. 16C shows a shape in which a side 62 a 1 and a side 62 a 2, which are curved lines forming the concave square, are not directly connected, and the side 62 a 1 is connected to the side 62 a 2 via a chamfered portion 62 b that is chamfered with a straight line.

In the case where the curved sides forming the concave square are connected via the chamfered portion, an intersection 62 c of hypothetical curved lines extended respectively from the side 62 a 1 and the side 62 a 2 as shown by a dotted line in FIG. 16C is defined as a vertex.

FIG. 16D shows a shape in which a side 64 a 1 and a side 64 a 2, which are curved lines forming the convex square, are not connected each other, and the side 64 a 1 is connected to the side 64 a 2 via a chamfered portion 64 b that is chamfered by a straight line.

In the case where the curved sides forming the convex square are connected each other via the chamfered portion, an intersection 64 c of hypothetical curved lines extended respectively from the side 64 a 1 and the side 64 a 2 as shown by a dotted line in FIG. 16D is defined as a vertex.

Whether the cross section formed by curved lines is a convex square or a concave square can be determined by hypothetically depicting a square by connecting the vertices (intersections 62 c or intersections 64 c).

The chamfered portion is not limited to the one chamfered with a straight line, but may be one chamfered with a curved line.

In the honeycomb filter according to the present embodiment, the thickness of the cell wall 63 separating the first exhaust gas introduction cell 62 and the exhaust gas emission cell 61 is smaller than the thickness of the cell wall 63 separating the second exhaust gas introduction cell 64 and the exhaust gas emission cell 61.

The following will describe the thickness of the cell walls in the cross section of the honeycomb fired body 60 according to the sixth embodiment shown in FIG. 15 based on the aforementioned definition of the cell walls. Supposing that a straight line Z₆₂ connecting the center of gravity O₆₁ of the exhaust gas emission cell 61 and the center of gravity O₆₂ of the first exhaust gas introduction cell 62 is given, the thickness of the cell wall 63 at a part overlapped with the straight line (thickness between the side 62 a and the side 61 a) is determined as thickness X₃. Supposing that a straight line Z₆₄ connecting the center of gravity O₆₄ of the second exhaust gas introduction cell 64 and the center of gravity O₆₁ of the exhaust gas emission cell 61 is given, a thickness of the cell wall 63, which separates the second exhaust gas introduction cell 64 and the exhaust gas emission cell 61, at a part overlapped with the straight line Z₆₄ (distance between a vertex 64 e of the second exhaust gas introduction cell 64 and a vertex 61 e of the exhaust gas emission cell 61) is determined as thickness Y₃.

In the honeycomb fired body 60 according to the present embodiment, the thickness X₃ of the cell wall 63 separating the first exhaust gas introduction cell 62 and the exhaust gas emission cell 61 is smaller than the thickness Y₃ of the cell wall 63 separating the second exhaust gas introduction cell 64 and the exhaust gas emission cell 61.

In the honeycomb filter according to the present embodiment, the thickness of the cell wall separating the first exhaust gas introduction cell 62 and the second exhaust gas introduction cell 64 may be determined in she same manner as the thickness of the cell wall separating the first exhaust gas introduction cell 62 and the exhaust gas emission cell 61.

In the honeycomb fired body 60 shown in FIG. 15, the thickness of the cell wall 63 separating the first exhaust gas introduction cell 62 and the second exhaust gas introduction cell 64 is uniform, and is equal to the thickness of the cell wall 63 separating the first exhaust gas introduction cell 62 and the exhaust gas emission cell 61.

In the honeycomb filter according to the sixth embodiment of the present invention, as shown in FIG. 15, the thickness of the outer wall is uniform; the shape of each first exhaust gas introduction cell adjacent to the outer wall may be the same as that of each first exhaust gas introduction cell not adjacent to the outer wall; and the shape of each exhaust gas emission cell adjacent to she outer wall may be the same as that of each exhaust gas emission cell not adjacent to the outer wall. Alternatively, the exhaust gas emission cells adjacent to the outer wall may be partially deformed such that the side adjacent to the outer wall is aligned with the line connecting the outermost points on the inner walls of the first exhaust gas introduction cells adjacent to the outer walls, so that the thickness of the outer wall except for the corner portions is substantially uniform.

In the honeycomb filter according to the sixth embodiment of the present invention, the thickness of the outer wall may be uniform in accordance with the shapes of the exhaust gas introduction cells adjacent to the outer wall, and the shapes of the first exhaust gas introduction cells and the exhaust gas emission cells adjacent to the outer wall may be the same as the exhaust gas emission cells not adjacent to the outer wall, respectively. In other words, in this case, the outer wall is bent in accordance with the shapes of the first exhaust gas introduction cells and the exhaust gas emission cells to maintain the uniform thickness.

The honeycomb filter according to the present embodiment can be manufactured in the same manner as in the first embodiment of the present invention, except that a die having a different shape is used in the extension molding process.

Hereinafter, the effects of the honeycomb filter according to the sixth embodiment of the present invention are listed.

The honeycomb filter described in the first embodiment of the present invention has a feature that the side 12 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the first exhaust gas introduction cell 12 is longer than the side 14 a facing the exhaust gas emission cell 11 among the sides forming the cross sectional shape of the second exhaust gas introduction cell 14. Meanwhile, the honeycomb filter according to the sixth embodiment has a feature that the thickness of the cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells is smaller than the thickness of the cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells. The fifth embodiment is different on the above points from the first embodiment. Other features are substantially the same.

A smaller thickness of the cell walls may lead to easier passage of exhaust gas through the cell walls so that the pressure loss may be reduced. Thus, the length of the sides forming the cross sections of the cells may correspond to the thickness of the cell walls separating the cells. Hence, the honeycomb filter according to the sixth embodiment of the present invention can exert the same effects as the effects (1), (2), (4), and (6) to (10) described in relation to the first embodiment.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise then as specifically described herein. 

1. A honeycomb filter comprising: a plurality of cells through which exhaust gas is to flow and which include exhaust gas introduction cells and exhaust gas emission cells, the exhaust gas introduction cells each having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side, the exhaust gas emission cells each having an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side, the exhaust gas introduction cells and the exhaust gas emission cells each having a uniform cross sectional shape except for a plugged portion in a cross section perpendicular to the longitudinal direction of the plurality of cells thoroughly from the end at the exhaust gas introduction side to the end at the exhaust gas emission side; porous cell walls defining rims of the plurality of cells, each of the exhaust gas emission cells being adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls; the exhaust gas introduction cells including first exhaust gas introduction cells and second exhaust gas introduction cells, each of the second exhaust gas introduction cells having a cross sectional area larger than a cross sectional area of each of the first exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells; each of the exhaust gas emission cells having a cross sectional area equal to or larger than the cross sectional area of each of the second exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells; and each of the first exhaust gas introduction cells having the cross sectional area equal to or smaller than 0.7 mm² in the cross section perpendicular to the longitudinal direction of the plurality of cells.
 2. The honeycomb filter according to claim 1, wherein the first exhaust gas introduction cells each has the cross sectional area equal to or larger than 0.5 mm² in the cross section perpendicular to the longitudinal direction of the plurality of cells.
 3. The honeycomb filter according to claim 1, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas introduction cells and the exhaust gas emission cells each have a polygonal shape, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side forming a cross sectional shape of each of the first exhaust gas introduction cells faces one of the exhaust gas emission cells, a side forming a cross sectional shape of each of the second exhaust gas introduction cells faces one of the exhaust gas emission cells, and the side of each of the first exhaust gas introduction cells is longer than the side of each of the second exhaust gas introduction cells, or a side forming a cross sectional shape of each of the first exhaust gas introduction cells faces one of the exhaust gas emission cells, and none of sides forming a cross sectional shape of each of the second exhaust gas introduction cells faces the exhaust gas emission cells.
 4. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas introduction cells and the exhaust gas emission cells each have the polygonal shape, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, length of the side forming the cross sectional shape of each of the second exhaust gas introduction cells is not more than 0.8 times length of the side forming the cross sectional shape of each of the first exhaust gas introduction cells.
 5. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells are each octagonal, the first exhaust gas introduction cells are each square, and the second exhaust gas introduction cells are each octagonal.
 6. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional area of each of the second exhaust gas introduction cells is equal in size to the cross sectional area of each of the exhaust gas emission cells, and wherein the cross sectional area of each of the first exhaust gas introduction cells is 40 to 70% size of the cross sectional area of each of the second exhaust gas introduction cells.
 7. The honeycomb filter according to claim 3, wherein the porous cell walls separating the plurality of cells have a uniform thickness in any part of the honeycomb filter.
 8. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells have octagonal cross sections, the first exhaust gas introduction cells have square cross sections, and the second exhaust gas introduction cells have octagonal cross sections, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional shape of each of the second exhaust gas introduction cells is congruent with a cross sectional shape of each of the exhaust gas emission cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are arranged in a manner that the exhaust gas emission cells are each surrounded by alternately arranged four pieces of the first exhaust gas introduction cells and four pieces of the second exhaust gas introduction cells across the porous cell walls, provided that a hypothetical segments connecting geometric centers of gravity of octagonal cross sections of the four pieces of the second exhaust gas introduction cells surrounding a reference exhaust gas emission cell of the exhaust gas emission cell are given, an intersection of the two segments crossing the reference exhaust gas emission cell is identical with a geometric center of gravity of an octagonal cross section of the reference exhaust gas emission cell, and four segments not crossing the reference exhaust gas emission cell form a square, and midpoints of respective sides of the square are identical with geometric centers of gravity of square cross sections of the four pieces of the first exhaust gas introduction cells surrounding the reference exhaust gas emission cell, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side facing one of the first exhaust gas introduction cells across a first cell wall among sides forming the cross sectional shape of one of the exhaust gas emission cells is parallel to a side facing one of the exhaust gas emission cells across the first cell wall among sides forming the cross sectional shape of one of the first exhaust gas introduction cells, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side facing one of the second exhaust gas introduction cells across a second cell wall among sides forming the cross sectional shape of one of the exhaust gas emission cells is parallel to a side facing one of the exhaust gas emission cells across the second cell wall among sides forming the cross sectional shape of one of the second exhaust gas introduction cells, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side facing one of the second exhaust gas introduction cells across a third cell wall among sides forming the cross sectional shape of one of the first exhaust gas introduction cells is parallel to a side facing one of the first exhaust gas introduction cells across the third cell wall among sides forming the cross sectional shape of one of the second exhaust gas introduction cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, distances between parallel sides are the same.
 9. The honeycomb filter according to claim 3, wherein an outer wall has corner portions, and wherein a side, which contacts the outer wall, of each of the exhaust gas introduction cells and the exhaust gas emission cells adjacent to the outer wall is straight and parallel to a side corresponding to an outer periphery of the outer wall in the cross section perpendicular to the longitudinal direction of the plurality of cells, in a manner that a thickness of the outer wall is uniform except for the corner portions.
 10. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are all square.
 11. The honeycomb filter according to claim 10, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional area of each of the second exhaust gas introduction cells is equal in size to the cross sectional area of each of the exhaust gas emission cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional area of each of the first exhaust gas introduction cells is 40 to 70% size of the cross sectional area of each of the second exhaust gas introduction cells.
 12. The honeycomb filter according to claim 10, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells have square cross sections, the first exhaust gas introduction cells have square cross sections, and the second exhaust gas introduction cells have square cross sections, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional shape of each of the second exhaust gas introduction cells is congruent with the cross sectional shape of each of the exhaust gas emission cells, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are arranged in a manner that the exhaust gas emission cells are each surrounded by alternately arranged four pieces of the first exhaust gas introduction cells and four pieces of the second exhaust gas introduction cells across the porous cell walls, provided that a hypothetical segments connecting geometric centers of gravity of the square cross sections of the four pieces of the second exhaust gas introduction cells surrounding a reference exhaust gas emission cell of the exhaust gas emission cells are given, an intersection of the two segments crossing the reference exhaust gas emission cell is identical with a geometric center of gravity of a square cross section of the reference exhaust gas emission cell, and four segments not crossing the reference exhaust gas emission cell form a square, and midpoints of respective sides of the square are identical with geometric centers of gravity of the square cross sections of the four pieces of the first exhaust gas introduction cells surrounding the reference exhaust gas emission cell, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side facing one of the first exhaust gas introduction cells across a first cell wall among sides forming the cross sectional shape of one of the exhaust gas emission cells is parallel to a side facing one of the exhaust gas emission cells across the first cell wall among sides forming the cross sectional shape of one of the first exhaust gas introduction cells, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a side facing one of the second exhaust gas introduction cells across a third cell wall among sides forming the cross sectional shape of one of the first exhaust gas introduction cells is parallel to a side facing one of the first exhaust gas introduction cells across the third cell wall among sides forming the cross sectional shape of one of the second exhaust gas introduction cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, distances between parallel sides are the same.
 13. The honeycomb filter according to claim 10, wherein an outer wall has corner portions, and wherein a side, which contacts the outer wall, of each of the exhaust gas introduction cells and the exhaust gas emission cells adjacent to the outer wall is straight and parallel to a side corresponding to an outer periphery of the outer wall in the cross section perpendicular to the longitudinal direction of the plurality of cells, in a manner that a thickness of the outer wall is uniform except for the corner portions.
 14. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, vertex portions of the polygonal shape are formed by curved lines.
 15. The honeycomb filter according to claim 3, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are point-symmetrical polygons each having not more than eight sides.
 16. The honeycomb filter according to claim 1, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas introduction cells and the exhaust gas emission cells are each in a shape formed by a curved line, wherein the porous cell walls include first porous cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells, and second porous cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, and wherein a thickness of each of the first porous cell walls is smaller than a thickness of each of the second porous cell walls.
 17. The honeycomb filter according to claim 16, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas introduction cells and the exhaust gas emission cells are each in a shape formed by a curved line, and wherein the thickness of each of the first porous cell walls is 40 to 75% the thickness of each of the second porous cell walls.
 18. The honeycomb filter according to claim 16, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells are round.
 19. The honeycomb filter according to claim 16, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas emission cells and the second exhaust gas introduction cells each have a convex square cross section formed by four outwardly curved lines, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the first exhaust gas introduction cells each have a concave square cross section formed by four inwardly curved lines.
 20. The honeycomb filter according to claim 16, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional area of each of the second exhaust gas introduction cells is equal in size to the cross sectional area of each of the exhaust gas emission cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the cross sectional area of each of the first exhaust gas introduction cells is 40 to 70% size of the cross sectional area of each of the second exhaust gas introduction cells.
 21. The honeycomb filter according to claim 1, wherein the exhaust gas introduction cells consist only of the first exhaust gas introduction cells, and the second exhaust gas introduction cells each having the cross sectional area larger than the cross sectional area of each of the first exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells.
 22. The honeycomb filter according to claim 1, wherein the honeycomb filter comprises a plurality of honeycomb fired bodies, wherein each of the plurality of honeycomb fired bodies has the exhaust gas emission cells, the first exhaust gas introduction cells, and the second exhaust gas introduction cells, wherein each of the plurality of honeycomb fired bodies has an outer wall on an outer periphery of each of the plurality of honeycomb fired bodies, and wherein the plurality of honeycomb fired bodies are combined with one another by adhesive layers residing between the plurality of honeycomb fired bodies.
 23. The honeycomb filter according to claim 1, wherein the honeycomb filter comprises honeycomb fired bodies, and wherein the honeycomb fired bodies include one of silicon carbide and silicon-containing silicon carbide.
 24. The honeycomb filter according to claim 1, wherein the porous cell walls have a thickness of 0.10 to 0.46 mm.
 25. The honeycomb filter according to claim 1, wherein the porous cell walls have a porosity of 40 to 65%.
 26. The honeycomb filter according to claim 1, wherein the porous cell walls have pores having an average pore diameter of 8 to 25 μm.
 27. The honeycomb filter according to claim 1, further comprising: a periphery coat layer provided on a periphery of the honeycomb filter.
 28. The honeycomb filter according to claim 1, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the first exhaust gas introduction cells, the second exhaust gas introduction cells, and the exhaust gas emission cells each have a uniform cross sectional shape except for the plugged portion thoroughly from the exhaust gas introduction side to the exhaust gas emission side, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a cross sectional shape of each of the first exhaust gas introduction cells is different from a cross sectional shape of each of the second exhaust gas introduction cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a cross sectional shape of each of the exhaust gas emission cells is different from the cross sectional shape of each of the first exhaust gas introduction cells.
 29. The honeycomb filter according to claim 1, wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, a cell unit is two-dimensionally repeated in a manner that the first exhaust gas introduction cells and the second exhaust gas introduction cells surrounding each of the exhaust gas emission cells in the cell unit are shared between adjacent cell units, wherein the cell unit has a cell structure such that each of the exhaust gas emission cells is adjacently surrounded fully by the exhaust gas introduction cells across the porous cell walls, the exhaust gas introduction cells includes the first exhaust gas introduction cells and the second exhaust gas introduction cells, each of the second exhaust gas introduction cells has the cross sectional area larger than the cross sectional area of each of the first exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells, and each of the exhaust gas emission cells has the cross sectional area equal to or larger than the cross sectional area of each of the second exhaust gas introduction cells in the cross section perpendicular to the longitudinal direction of the plurality of cells, and wherein, in the cross section perpendicular to the longitudinal direction of the plurality of cells, the exhaust gas introduction cells and the exhaust gas emission cells have one of a first structure such that the exhaust gas introduction cells and the exhaust gas emission cells each a polygonal shape, a side forming a cross sectional shape of each of the first exhaust gas introduction cells faces one of the exhaust gas emission cells, a side forming a cross sectional shape of each of the second exhaust gas introduction cells faces one of the exhaust gas emission cells, and the side of each of the first exhaust gas introduction cells is longer than the side of each of the second exhaust gas introduction cells, or the exhaust gas introduction cells and the exhaust gas emission cells each have a polygonal shape, a side forming a cross sectional shape of each of the first exhaust gas introduction cells faces one of the exhaust gas emission cells, and none of sides forming a cross sectional shape of each of the second exhaust gas introduction cells faces the exhaust gas emission cells, and a second structure such that the exhaust gas introduction cells and the exhaust gas emission cells are each in a shape formed by a curved line, the porous cell walls include first porous cell walls separating the first exhaust gas introduction cells and the exhaust gas emission cells, and second porous cell walls separating the second exhaust gas introduction cells and the exhaust gas emission cells, and a thickness of each of the first porous cell walls is smaller than a thickness of each of the second porous cell walls. 